For almost every type of reservation, DCL holds some spaces so that people can get reservations after they get onboard. So I would first recommend that you stop at Guest Services shortly after boarding and ask if there are any spaces left for the Princess Gathering.
Also, the princesses will make other appearances around the ship. They just won't be all together in one spot, like they are at the Gathering. Check your Daily Navigator.
When my daughter was young enough to be in the Oceaneers' Club, many characters, including princesses, visited the club and played games or read stories with the kids. So if your child will be in the Club, s/he may get the chance to interact with them there.