TWO MAIN POINTS:

In-person appointments can be scheduled if the child is turning 21 in two weeks or sooner by contacting the  USCIS Contact Center to request an expedited appointment with a USCIS field office .

If you try to make an appointment more than 14 days prior to the child’s twenty-first birthday USCIS will presumably advise you to file by mail instead. 

No I-360 receipt will be issued at the in-person appointment.

The I-360 receipt will be issued by mail but will have the filing date of the day they appeared in-person at the field office. The USCIS field office won’t issue any proof of filing at the appointment.