I like the (for some reason downvoted) reasonable suggestion by someone else to simply not monetize videos containing children. Doesn't YouTube already have some kind of classifier that finds children in videos, which lets them turn off commenting on those videos? Just extend that to also de-monetize them.
Sure, it doesn't solve the problem of child videos using other monetization channels like product placement, sponsorships and so on but don't let perfect be the enemy of good.