To be honest, I think children adopted very early in life would have better childhoods than children raised by their birth parents because

p(parents are generous/caring/loving | parents are the type of person to adopt a child) > p(parents are generous/caring/loving | parents bring pregnancy to term)

In addition, for adopted parents, we know that both parents want the child. Whereas with birth parents, we know that the mother wanted a child but we do not know if the father did.