“Foster” is variously defined as “substitute,” “temporary,” or “short-term.” So a “foster child” is an actual human being who is a “substitute,” “temporary,” or “short-term.” The antonym is “natural,” so a foster child is also not natural.

We met N (No One’s Child) after her mother could no longer care for her. As best I recall, N was about ten years old and a friend of one of our daughters in school. Both were young girls, both were in the same class, both were smart and ambitions. There was one slight difference. N was temporary and our daughter was permanent.