Is it illegal to use your husbands surname for a child that is not his and not on birth certificate?

You may call your kids anything you want, but you can not use that name for school or change it on the birth certificate.

So I may call my poodle a pt bull all day, it does not make him one.

It wlll only of course confuse the kids as to their real names.

Also if this is part of turning the kids against their bio father, he can use that in court

As long as you are married legally,it`s yours,too. So,no,it`s not illegal.
(My sis is living w/the papa of her youngest,who carries her ex-husbands name,
Since it`s legally her name it`s all good.)

I think this would be better answered in the legal boards.