Sounds great! Except all too many parents are incapable of being responsible for their own kids, either out of apathy or economic inability.
So when you do that, a perfectly well-meaning but stretched family might decide to, say, live in a house with heavy lead contamination. Which, aside from the heavily detrimental effect on the kids, has very large externalities, as children who have been exposed to lead in high concentrations are much more likely to become criminals.
This assumes there are no consequences for such parents. Add strong enough consequences (e.g. loss of freedom) to deter others. A "well-meaning but stretched family" is more likely a set of parents who didn't responsibly plan their family. If that's the case the parents should be punished. (In the US, even the poorest parents can be punished for exposing their kids to lead.)
If we make parenting a birthright (as many people support, esp. for the poorest for some reason), mass starvation is the inevitable result.
So when you do that, a perfectly well-meaning but stretched family might decide to, say, live in a house with heavy lead contamination. Which, aside from the heavily detrimental effect on the kids, has very large externalities, as children who have been exposed to lead in high concentrations are much more likely to become criminals.