There is very little you can do at this time. Hitting send is like dropping the return in the mailbox. Just wait until the IRS either accepts or rejects your return. If they reject it, then you can just fix the problem that caused the rejection and add the dependent. If they accept it you will have to amend your return. The forms for amending will not be ready for a while (usually it is in the middle of February).
Whatever you do, DO NOT make any changes to your filed return until you hear from the IRS.