Your exhusband must be given notice of any suit to change the names of his children. That notice should be the same as in any other lawsuit. I suggest you contact an attorney to help you do this. Attorneys often have ways to locate people that don't come to mind ordinarily.
I am not intending this to be legal advice, because I don't know the particulars of your situation. Call me if you would like to discuss this or other isues.
Thank you for your question. I hope that you have a law firm helping you with the termination of parental rights, which, should also be helping you with the name change. Legal matters are complicated and should be handles by professionals. Good luck and God bless you.