More commonly, 824 is to...

More commonly, 824 is to 1) request a duplicate of an approval notice, if the notice is missing. 2) notify a consulate an approved application/petition, or approved 485 of your case, so that the consulate can approve your your family member's case.

Since your kid is in US, I don't see the purpose of using 824. Your goal is to add your kid's name on future waiver notice. Maybe your lawyer has better understanding of using this particular form.