Because the cost of raising a child to 18 years of age far outweighs the $10k + interest?

An increase in income is generally tied to having less children, not more. I read a study (which I can't find right now) that studied the effect of policies that gave income based on the amount of children, and there is no increase on the amount of children people have. Argentina, for instance, has a program called Asignación Universal Por Hijo that (roughly) pays you for each child you have, provided he goes to school.

Only the social security number holder at 18 has access to the funds. I don't think it would be too hard to include methods to prevent abuse. I may too idealistic.