Bikes are children's toys, that's so... Saudi Arabia?
I really consider this a terrible design principle, because nobody likes to be treated like a seven year old and doing so could be a serious disincentive against cycling for anyone but the most irrationally fearsome. Design for competent cyclists, everyone can eventually become one. If the result happens to good for the seven year old, fine, it'd not that unlikely if money and space permits it at all. If not, treat children like pedestrians and grant the appropriate exceptions in law for supervising adults. They are surely not the kind of cyclist who causes havoc on the sidewalk today (this won't work in exceptionally walked places like most of Manhattan, but there, nothing less than a complete network of elevated bikeways could be made even remotely child-proof while providing real transport and not just some recreational riding in a park).