you only file your return claiming your son - no documentation that you are entitled to claim him. if she files first you will have to mail in your return. then in a year or so. more or less, the IRS will send a notice to each of you to amend. you don't respond. if she too doesn't respond there's a second letter to each where the IRS ask for proof that the filer is entitled to claim. the letter will list documents that will serve as proof. That she won't return your child is a legal issue.
