There's a lot of non-profits that aim to make birth control more wide-spread, too. Do you have any other ideas how overpopulation can be tackled by a startup with purely legal methods?
I believe that decreased child mortality and increased education/self-sufficiency for women are two of the big triggers for reduced birth rates in developing countries; in other words, increase the chances that the kids they have will survive to support their parents, and decrease the need for mothers to be supported by their children/possibly increase the opportunity cost to them of having children (although I don't think I've heard it described in that last way, it's just how it looks to me).
