The point of the one-child policy is to decrease the rate of population increase by decreasing the rate of child births occurring. Since foster children have already been born, not adopting them wouldn't do anything in regards to the population size. To put this in another way, foster children are still a part of the population whether you adopt them or not.
This makes absolutely no sense. If you are trying to reduce overpopulation and more than just overpopulation but the negative effects of overpopulation such as homelessness and poverty then you should make adoption less expensive. Families who choose to adopt a lot of children should get bonuses. Families that forgo having kids altogether and only adopt should get even higher bonuses.
Note I'm not saying I support the one child policy, just that if you're going to do it do it right. You want to curb overpopulation, adopting kids does not contribute to overpopulation, it alleviates the problems caused by overpopulation and can reduce the population growth rate if people have less kids or no kids because they chose to adopt.