shlishi: There are some cases where Jewish children are available for adoption, but they are very few. Most times, when a Jewish couple adopts someone, it is a non-Jewish baby. They will usually do a geirus as soon as possible. At bar/bat mitzvah, the child has to decide whether they want to stay Jewish or not.
I heard of one boy who, at bar mitzvah, said he didn’t want to stay Jewish. He said he believed in the Torah, but didn’t want to be a Jew. He keeps the 7 mitzvos b’nei Noach, and is a good person. He said, why would I want all these rules when I can be a good person and get olam haba as a non-Jew?