If you had the baby in 2019 then you cannot add the baby.
If you had the baby in 2018 then you can file an extension and wait to file the tax return until you receive the Social Security card. Or you can file without the baby and file an amended tax return when you receive the card.
https://ttlc.intuit.com/replies/3301811
https://ttlc.intuit.com/replies/7276605
