Once your return is filed and "Accepted", you can't make changes. If the return is Rejected, you can correct the error by deleting that child as a dependent and re-entering his/her information in the Personal Info interview.
The birthdate of dependent children is used to determine eligibility for certain exemptions and credits (e.g. a child must be age 12 (or younger) to qualify for the Child Care Credit).
Unless the error was big enough to create a problem with an exemption or a credit, it won't affect your return. Birthdates are not used for purposes of identification on e-filed or paper-filed returns.
