Going invisible to observe the children secretly and losing the parma on the grogs vs risking the kids noticing him but maintaining the parma is a tactical decision.
But the other members of the group could still see each other.
. It does not say that Inigo has to be the see-er or the seen, just that any one of them can see the other. If the magus needed to see them or be seen, then I think the choice of wording would have been much different. I am not gonna add unwritten restrictions.
But there is a thread about it on the main Forum if you would like to get feedback from the community. I started it asking about this very situation. I often like to use that as a resource.
OOC: That's just weird with the Parma Magica. I need to look at the older editions to see if that applied.
I have a problem with the line-of-sight issue. Why? Apprentices. Under that logic, an Apprentice would cease to be protected by his or her Parens/Mater's Parma the instant the two of them were in different rooms. This would greatly limited the effectiveness of an Apprentice in the Lab or in everyday life at a Covenant, when the Parens would expect them to run errands yet, according to the customs of the Order, is supposed to also protect them with their Parma. At a Tribunal, where the Apprentice would be highly vulnerable to other Magi and/or magical forces (wielded by those not actually sworn to the Code), the Apprentice would be effectively "naked" the moment he or she left sight, which would be bound to happen in negotiations, deals, or private discussions.
Do you see my logic? If that restriction holds, sharing Parma Magica with Apprentices--never mind shield Grogs--would be that much less viable. This in an Edition that treats charms against magic (a staple of ALL prior editions) as anathema.
Nah, just cover a pair of grogs as well, and lock them in a room together. Then you and your apprentice can range as far apart as you desire 
I don't recall ever seeing something in regards to an obligation of having to extend the parma magica on the apprentice. Of course it's a prudent thing to do. It's also prudent not to drink alcohol but people do it anyway. /shrug
HoH: TL Bonisagus section spoke of the creation of charms against magic as an example minor breakthrough for characters to develop. While I'm not well versed in the editions prior to 4th, I don't recall seeing any significant text or sections about charms against magic.
I agree with the point that Line of Sight is a bit silly of a restriction in the first place. Which falls under my dislike about Ranges in 5th edition. Not saying 4th edition ranges were perfect. I wasn't particularly fond that Reach and Touch are two separate ranges. IMO it should have gone Per -> Touch/eye and skip to Near. Reach just seemed tacked on to add another magnitude level for spells.
Personally, I'd have gone for something where the recipients are under the effects of the parma based on proximity like the old range "Near" which is plenty flexible to cover a situation where say the two characters are fighting back to back while surrounded.
An interesting option is to give each member of the protected group some kind of token, protection lasts as long as you keep hold of it (or until it is retracted). This would serve as a sort of weak two-way Arcane Connection.
Falls, I don't wan't to make a ruling that I will later regret, and since it would be wise and prudent, perhaps you should pull your full protection back over yourself. Metron is about to emerge from the Twilight Void at any moment to launch a surprise attack.
Sounds good to me.
Okay, you look around and there is still no sign of Metron anywhere. The children yu had been flying after, they had stopped to watch the fight and are looking around confused wondering where the bad guy went off to. To them, it is like watching wrestling on TV 
Octavian, Alexandro is asking permission to investigate outside. The Parma has been withdrawn, and the men start feeling uneasy because of the gift. But they are disciplined and professional soldiers. Alexando has gotten used o it, as he has a father and sister that each have the Blatant Gift.
Vares, you are making your observations, and there is one important distinction you need to be made aware of. This house is not the actual Magic Realm, it is a level 10 regio. Outside is the Magic Realm. This cottage was apparently built by a Verditius using Hermetic Architecture.
Octavian approaches the custos at the request, testing their reaction to his gift as if to taunt them. "Your request is denied."
He gives you a look as if to say you made an unwise decision, but he will abide by it. He posts a man at each door and sends the third upstairs to keep a lookout.
OOC: The rule about covering an Apprentice with Parma Magica is, I think, in the section on Apprentices in the main book.
Previous editions allowed the diminished Parma Magica to be in effect at any range, but one only gained the extra bonuses of the Magus' Form scores when within 15 paces (the old Near Range). So it was stronger when right next to the Magus, but had some effect even at great distances.
OOC: Thanks for the note regarding the house.
Since he hasn't been briefed on the encounter with Metron, Vares is going to continue his own investigations. Specifically, focusing on the threshold of the house and the terrain outside it. He'll measure astrological time at the interface, and also spell effects cast from within the building to the outside, as well as vice versa. Minor CrIg spells, same as before.
You are using some high powered Intellego Vim spells I take it. and you have second sight.
Check this out...
You are able to see into the moon, and you think that if you could fly up there, you could travel through it...
And at that moment, Ludovico pops out of a wormhole.
Yes, Ludovicio, whom you though was missing, was lost in the Twilight Void for the past twenty-four hours. He is holding one of the house keys, and pops into existance on the front porch.
OOC: What does Vares witness at the moment Luddoviccio returns? With "Strings of the Arcane Connection" in effect he could probably perceive something very interesting on the "other side" of the Twilight Void.
More to the point, could he distinguish between that and an ordinary teleport spell?
Vortigern and I have been planning things secretly since he got back stateside. You are utterly surprised, and he seems just as surprised. No AC to detect, except for the key he is holding. It is one of the guest keys to the house here. Ludo was almost lost for all eternity, and he thinks it is still yesterday.
To clarify, no teleportation was used that you can detect. You don't know how Ludo got into the Twilight Void, but it was second sight that led him here. What you observed was similar to watching someone cross a Regio level.
There is no sign that the criticism has impacted the Tremere as he addresses the woman. "You may stay here as long as you do not leave."
He quietly makes his way to the 2nd floor and stalks the walls of the house, casting multiple castings of the spell Prying Eyes to keep an eye out on what is transpiring outside. With the bonus from the aura, he'll cast it silently and without gestures.
OOC:
Can I get a brief recap of the current situation as is perceptible to Ludo?
Inigo is flying around in battle with Metron? Is this visible to Ludo? Can Ludo see anyone else from the group? Or anyone at all for that matter? Hear anything etc?
Does Ludo's spiritual retinue come through this rift with him, or no?
Since he was 'investigating' and in 'adventure' mode when this mishap/transportation occured, if there are no objections I'd like to assume he had his sustainable spell repertoire already up and in place, to include some basic rego sponts similarly sustained to protect/ward him from mundane corpus/animal/terram/herbam type stuff. Nothing fancy that would stop a spell, just enough to turn aside mundane attackers.
IC:
A bubble shaped ripple in space expands to offer a brief glimpse of another landscape which is obscured by a flashing explosion of magical energy which is suddenly also obscured by a human-shaped shadow reeling backwards away from the event. Then the bubble contracts and fades into nothingness.
Blinking for a moment, then lowering his hands, Ludo stands surveying his surroundings. He is dressed in his 'traveling' clothes, which though they nominally conceal his face and identity, everyone in the group has seen him wear in transit from Andorra to Barcelona. Black robes with blue and white underlayers and embroidery, a contrast to his usual venetian finery. At first his eyes are wide in surprise, one hand coming to rest on the short sword at his hip as his gaze turns back and forth. In the long moment afterwards, no immediate threat materializing, his expression becomes more wary and dark, rather than shocked. [color=red]"Damn that man." he says softly. [color=red]"If I see him again he will regret it."
Vares, I should not have told you about the Arcane Connection, your spell would not Penetrate Ludo’s Vim Resistance. I was thinking to myself out loud. But you do see him sort of materialize as he steps forward, like he just crossed through from the other side of nowhere.
Ludo, you just came out of the Mentem Province of the Twilight Void, and you are in rough shape. All of your spells that were active beforehand are ended and your Parma is down too. None of your spirit entourage is at attendance. What you did was extremely dangerous, and you got extremely lucky. I will talk to you about it more on the side later.
Inigo, the tension is thick. Your enemy is out there somewhere, you are sure of it.