I'm mad keen on the ArM setting and magic system. I also love the idea of covenant-centred play. However, I'm also wanting to play with my 8 year old daughter. She has experience with rules in the board game sphere, but I'm not sure that ArM rules as written are the best starting point for what I'm selling as the next step up from “let's pretend”.
I've seen the Wizard World PDF and the suggestions for a simplified experience on Timothy Ferguson's blog. I suppose that at a stretch I'm also ok with something completely free form. Rules are a comfort though, and we mostly had fun following the ArM rules to generate the characters. Is there anything else I should be aware of? Is there some resource out there that will make me confident in my ability to resolve situations in a rule-ish way, that won't bog me down, but will allow for dice to inject uncertainty and that little frisson of excitement about outcomes?
Really, really don't sweat the rules and details much at all.
1, Virtues & flaws don't have to balance.
Vis is vis, no techniques or forms.
3 don't get bogged down in any complicated rules, and strip anything down in the way of FUN. i.e. Spell mastery - a spell is mastered, or it is not.
I'm not sure how far you're going with the magic system, but it can be dramatically simplified just by focusing on the techniques and forms without going to too deep into the score of them.
Remove all division and multiplication and subtraction as much as possible. Just add numbers together.
I would go much further in term of simplification.
First, to simplify the game, you will have to do more front loading & preparation:
Gives her pool of colour beads/token for her magic. Two, at most 3 colours. Each colour represents one Form. The more powerful, lasting, broad the spell, the more beads she needs to spent to activate the effect - one bead per magnitude if you want to have link with Ars spell, but if you don't want too many beads, go with one bead for two magnitudes. When she is out of beads, she cannot use this magic for the day. Yes, Ars does not work like that but it shows her to manage her resources.
If you want to give her mastered spell, prepare a few cards with the spell on it, with a few circles representing the beads needed to activate it. Blackened some of these dots to decrease the spell's cost: if shows her mastery - less resources to spend, and she can spend more to amplify the effect. For example, "Pilum of fire, with 2 dots (=2 free magnitude"): by spending two points, she gets the base effect. By spending 3 points, she can do Ribbons of Fiery flame, etc...
With this two points, you manage both spontaneous magic and formulaic, without bothering with lots of number crunching.
For skills, use the same principles: two pools of points physical and mental.
She has skills, with a certain number - make them generic, no need to have five different social skills (Leadership, Guile, Etiquette, etc). She needs to spend a point to use a skill as whatever level she has, then can boost it by spending additional points. Those skills acts in a similar way to spell card: it gives a base effect (level of success), that can be further boosted.
Obviously, you need to change the scale of difficulty: 1: beginner up to 7 greatest master, and average difficulty should be 2, at most 3. If you want to introduce a random element, you roll a dice to represent a difficulty: 1d4 for something average, 1d6 for more difficult task, with +1, +2 modifier for extra difficulty.
I am working on this system for my own daughter, but she is still too young for that, so it is even more simplified.