What was the children's book where they lived in a society where they could only have 2 kids but the protagonist's family had 3 and the extra kid had to be kept hidden?

Answer 1

amomg the hidden by something (first name) haddix (last name) .

Answer 2

The book you are referring to is "Among the Hidden" by Margaret Peterson Haddix. In the story, third children are illegal due to a government law limiting families to two kids. Luke, the protagonist, is a third child who must live in hiding to avoid being discovered and arrested by the Population Police.