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All cultural assumptions made in your hypothetical aside...

(((You are saying that nobody will ever tell them it's abusive or their genitals are icky. And while you don't specify it, it would be easier for the hypothesis if we still assumed that it is a private, at-home, activity, not a public one (which of course makes it somewhat different from a hug, which isn't recognised as something that must always be private), or otherwise we'd be assuming that genital touching is ok in public and the hypothesis would be about society, making the individual child you seem to be talking about redundant)))

...so, in this society, or roughly in this society...

I've always wondered, if a parent spent a few minutes each day, from when they're a baby, stimulating their child's genitals, at what age would the child start to question or have a problem with it?

If it always stays at that level, and does not escalate into more and more forms of openly sexual activity as the child ages, I would expect two answers, mutually excluding in an instance-by-instance basis, but not mutually excluding as generalisable answers.

First - at the "teenage rebellion" stage.

Like every activity that any given set of parents and children have shared since infancy and that has always seemed all right and a family bonding moment, the "teenage rebellion" stage is bound to make the child resist it at least sometimes, in their effort to assert separate interests and identities from the familial one; as well as to "leave the childish stuff behind".

This, of course, will vary and be mediated by how many and how big reasons the child has to rebel out and how the parents deal with that rebellion anyway.

Second - never.

I doubt that it would be considered as trivial as a hug unless the whole society agreed that it could be practiced in public at least at times and did it - just as hugs.

However, like in the first answer, I think it can be regarded as just another parent-children family activity like anything else that is usually done at home and in private. And in that sense, I am sure that even during the teenage rebellion stage, children will at times want to do that kind of thing to feel their parents are still close to them (just they sometimes will say no along with the times they say yes and even ask for more). And while I'd expect the number of instances to reduce as they move out, I don't see why there wouldn't be a space for that even as adults, anymore than for a movie, a board game, a dinner or a fishing trip...