I have the same problem. We are taking my Dniece7 and Dnephew10. She is going to be Cinderella. I was trying to think of a t-shirt design for him. I was thinking something like "I may not be dressed as a prince, but my aunt thinks I'm one". Since I started thinking about it, he has said if I can put together a prince costume, he'll wear it.
I like the "so many princesses, so little time" idea! I think that would make a great shirt.