My thoughts are thus:
the child has been exposed to the parent and their gift their entire life, and thus would be able to ignore it (5 years constant exposure, aside from impressionability of youth)
the parent is not immune to their child's gift. Reactions vary, from some being eager to be rid of a child they don't understand to trying to punish the child to 'correct' the behavior which isn't the real problem. Hermetic magi however are not affected by the gift due to their own parma magica.
That's mostly my impression of the situation. The child of a Gifted mother will probably respond badly at first (lots of crying) but has few options, being completely helpless. By the time they can talk, mom (who will need the patience of a saint) has been the sole source of all their needs and they're probably mostly used to her Gift. Hermetic maga have a huge advantage in this regard, as covering the child with Parma is fairly routine.
However, my original point stands - that a group of Gifted children are going to have serious problems with each other. A grog can acclimate to a Gifted person over the course of years (canonically 15, less if the Gifted is making real efforts towards being nice), but an adult can rationalize a lot better. "My boss can't be trusted but I need to eat" is a good reason to put up with your boss (also, the boss can throw lightning bolts). Even if mom/dad will lay on you a beating for fighting with your brother, that's your brother's fault, because he's a dirty lying jerk who always starts the fights. The Gift can only make natural sibling squabbles far worse.
With a Gifted parent and child interaction, it's tough to say how long that period will last for a child to become used to the parent's Gift. There are other dynamics of the parent-child relationship that can affect how a child reacts to a parent trying to reasonably discipline them. The "Terrible Twos" might never end, and the child will always feel better being around someone other than their parent. So, the child, even when young, might run away to someone else. At some point it becomes intolerable for the Gifted parent to constantly go after the child from wherever they ran, and they then give up.
I guess my point is, that while it seems reasonable that the child could become used to a Gifted parent, it's also reasonable that the child may never get used to the Gifted parent. Assuming one position over the other ignores a lot of story potential.
A child who is responding poorly may reasonably fail to nurse from a Gifted mother, because they are uncomfortable. So, there is little alternative to make things right. Wet nurses are common in Mythic Europe, and so a lot of bonding will happen between the child and wet nurse that can't happen between the child maga. And yes, one can use Mentem magics on the child to ensure that they do nurse, but one wonders if they are willing to invent the spell or willing to accept the risks of botching a spell that is cast spontaneously, requiring fatigue...
Then there's the entire parent-child dynamic that happens in the pre-teen years with growing demands for independence, often culminating in rebellion during the teenage years. The Gift, even if the child is "used to" the parent won't make this any easier.
Keep in mind as well the Gift doesn't always manifest at birth. you may have known you brother or sister for 10 to 12 years when the gift manifests...
It's a good point. I'm sort of assuming any Gift-manufacturing scheme will end up with a manifested Gift at birth. The Apprentices book deals with this a bit; most Blatant Gifted newborns are abandoned, Gifted ones are grudgingly raised but considered ill-omened. Lots of people manifest the Gift later in life (puberty or even later).
If you are doing that you may as well give them the gentle gift. Remember this is based on your lab total, like a longevity ritual, and a lab can easily be specialized for creo corporum, plus there is the possibility of a fertility fetish (and if the maga is creating her own offspring the points double!)
A mother doing it for herself is the best case scenario; Gift + Gentle Gift is 6 trait points, which takes a CrCo26 (51 for someone else), which isn't hard to achieve, but this does preclude other Major Hermetic Virtues. I guess it depends how many traits you want to stack up, and how many children you want.
Also, know whether the SG is going to make sure the kids end up with balancing flaws or not.
A maga Bonisagus is likely to decide that gentling the Gift would not be worth any random issues that might cause.
(Also, I personally do not consider Gentle Gift a true Hermetic Virtue for purposes of the Major-Hermetic-Virtue thing, but that's me.)
according to the text they shouldn't. YSMV
Which text?
It is possible, under the rules of Apprentices to instill multiple Hermetic Virtues into apprentices, so long as they can reach the requisite SQ. Under the core book magi should not be designed with more than one Hermetic Virtue. Both circumstances can be true in the case of Apprentices.
Ancient magic- p. 58 "The virtues and flaws determined by the fertility ritual do not need to balance... if the child is a player character... allows the child to develop additional virtues and flaw as he ages." which means that according to AM, the GM should not be assigning random flaws to balance traits gained through Fertility rituals.
Do not need to balance does not mean that the SG/troupe shouldn't assign flaws they deem necessary. And no SG should be assigning random flaws. But thematic flaws in line with some of the virtues so gained is certainly not explicitly verboten. And it is totally consistent with the development in Apprentices, that virtues and flaws may be gained as the child ages.
No, in fact it specifically makes reference to allowing the child to develop balancing flaws later in life, which also fairly explicitly indicates that they do not occur at birth. It also specifies that this should be done specifically for player characters produced in this way, not necessarily for NPCs. After all the point is not to spend vis pointlessly (since as a ritual it does require vis)
Can you point out where it says that? Apprentices states that Virtues and flaws need only balance at character generation.
Edited to add quote.
there there, no need to yell...
It's a quotation, actually copied and pasted to maintain fidelity from the source. I didn't yell, Apprentices yelled. 
Apprentices are always yelling. They're so ungrateful.
By my reading, this is the key sentence. Fertility Magic is intended to allow its practitioners to breed "superior" offsprings, not balanced ones. I wouldn't compensate for extra virtues with extra flaws.
Like the rest of the features of Ancient Magic, Fertility Magic is overpowering and game changing. That seems to be the point of the book. You shouldn't think of it as just another minor virtue.
I already did show where it states that, in the quote three posts above the one where you ask for the reference. I even gave book and page number.
Fertility Magic says a number of things
- Traits need not balance: It does not say they DO NOT balance, but merely that it is not a need. That doesn't mean that they CAN NOT balance.
- It recommends that for PCs (and only PCs) that by adulthood the traits do balance.
So its guidance, but it ultimately says the troupe can make their own decisions. Honestly though, I don't think its a huge deal if the PCs don't balance. Its a research project with a long payback. Other research projects can produce immediate benefit and there are other ways to gain similar power for adults. YMMV.