You cannot change or add anything on the return that you e-filed. It is too late, just like when you put an envelope in a US mailbox
Now you have to wait until the IRS either rejects or accepts your return after they open on Jan 27. If your return is rejected, you will be able to go into your account and add or change it and resend it.
If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund. THEN you can prepare an amended tax return 1040X and mail it in. You cannot e-file an amended return. Meanwhile, DO NOT change anything on your return until you start the amended return. If you did change or add the missing info then take it out and put your return back to the way it was when you first filed it.
How to amend an Online return
When was your baby born? A child born (even on Dec 31) is considered to live with you the whole year. So pick the Whole Year at the top of the drop down box.