We have adopted an infant.

Answer

The identity of an adopted child may not be concealed from him/her. As the child grows in understanding, he/she must be made aware of his/her true parents. It is not permissible to deceive the child into believing that the adoptive parents are the true parents. If changing the surname is for convenience, it will be permissible. But it may not be changed if the intention is to conceal the child's identity.