You need to wait for it. You cannot submit an accurate and complete return without claiming your child, and you cannot submit a return without having her SSN. You also cannot apply for a separate tax ID#, as the child is currently an SSN applicant.
Most eligible children born in 2016 have their SSNs by now. Have you made the appropriate follow-ups with the SSA?
Keep in mind that returns that show refunds aren't negatively impacted by the 4/18 due date. If you expect to owe or want to be on the safe side, however, you should definitely file an extension. Pay any estimated tax due with this extension and file the return(s) as soon as the SSN arrives.
