How do I go about adding my child’s w-2 after I’ve already filed?

[xmasbaby0]

You do not add your child's W-2 to your own tax return.  

MY DEPENDENT HAD A JOB

If your dependent has a W-2 for his after-school job, summer job, etc. you do not include the information on your own return. You can still claim your child as a dependent on your own return.  He/she can file his own return for a refund of some of his withheld wages (he won’t get back anything for Social Security or Medicare), but MUST indicate on it that he can be claimed as a dependent on someone else’s return.  (Supervise this closely or prepare it for him!)

If your dependent’s earnings were over $400 and were reported on a 1099Misc then he must file a return and pay self-employment tax for Social Security and Medicare.  You may want to use this version of TT for that:

https://ttlc.intuit.com/questions/1900583-what-is-turbotax-free-file-program

[DoninGA]

You do not enter a dependent child's W-2 on your tax return.

The child can file their own tax return and indicate on their return that they can be claimed as a dependent.