I just heard a talk from a PhD student on how anaemia in early life adversely affects both motor and cognitive abilities, so something about how adequate nutrition affects development.
Its well known that girls develop at a faster rate than boys so you could ask about the mechanisms behind this.
What effects do maternal factors have on the developing foetus? For example does a high fat maternal diet during pregnancy or lactation predispose a child to obesity?
This is a pretty broad question, so there are lots of things you could ask.
There is always the question of nature vs. nurture. (I say nurture)
I believe it can be both depending on what is being questioned.