What happens if the other parent has the social security number of our child will not give it to me and may have already filed

Which one of you is the custodial parent?  Who does the child live with?

if you are named on the birth certificate, you should be able to get the SSN from the Social Security office.  if you are,  call them for the procedure to get it becuase they will not give it out over the phone.

note the iRS doesn't care what the divorce decree provided.   generally the custodial parent is entitle to the dependent.   if the decree provided something else, you'll have to take the spouse back to court for violating the decree.    you'll also need to paper file yor return regardless of whether you claim the dependent or not.