About child last name

[Rich4bright]

hi everyone, I have sent my application and I changed my child last name to my new husband because the man I had the child for was never in the picture from birth. But I had to find him to sign the form for a non accompanying parent with a lawyer. But now immigration was asking me for reasons for changing the last name from my last name to my present husband’s name. The father of the child signed the affidavit to say he was in support of it. Could anyone help me pls? Thanks in advance