You must have a valid SSN for the child on an "on time" return in order to get any tax credits. If the child is eligible for an SSN, wait for it to arrive, even if you file late. Don't file without the child and don't use any other number for the child. You can file an "on time" tax return up until October 15, 2020, IF you request the automatic extension by midnight on July 15, 2020.
