Re: "If you were to look only at what sensory data children get, you would find that it is woefully insufficient to "train" them to the level they eventually reach, no matter what epistemology they're using."

That hasn't been demonstrated - AFAIK.

Children are not blank slates - but if they were highly intelligent agents with negligible a-priori knowledge, they might well wind up eventually being much smarter than adult humans. In fact that would be strongly expected - for a sufficiently smart agent.