You would just need to amend each years return that you made the mistake on. Just log back into your account and click Amend My Return. You will need to do each year separately and print and mail each year separate.
Before doing so, double check to make sure you did switch their social security numbers simply because they normally double check the SSN's with the Date of Birth before accepting the return. However, since it was on the same return, it may have slipped by.