A child was not in his hometown A when he was a child, but grew up in B. Everyone around him spoke the language of B. Could this child speak the language of B when he was a child?

It depends on the specific growth of the child. If all the children's family members speak A, then even if the children grew up in country B, they can only speak A before meeting a large number of people other than family members, such as before going to kindergarten.

The most obvious example is that after a child is born in a China family who emigrated, the child speaks Chinese with his parents at home and occasionally speaks a foreign language. After they went to kindergarten, they had more contact with society and became fluent in foreign languages.