We are in the same boat, we have a trip next month and have a 12 year old. We are fully vaccinated but obviously at 12 she is not. I assume they will release information about 5 days before the change.

We are trying to schedule our daughter for the shot on the first day it is allowed and with Pfizer you can technically have as little as 17 days between shots per scheduling guidance (21 is stnd) . So the potential window for first shot is about 4 and a half weeks before travel.