We did something similar -- Our kids birthdays were 2 weeks apart -- late sept and early oct and our trip was in Nov. My daughter turned 3 and my son turned 5. She, of course wanted something princess and he wanted something star wars. We reached a middle ground with an Aladdin theme. They both like that one because it has a little bit villian and a little bit princess. DD even wore her jasmine costume for the party. Anyway, we rented a projector to show the movie on a big screen -- the idea was to do it in the backyard, but it rained so we ended up inside showing it on a wall.
We mailed invitations and included tickets with aladdin and jasmine on them. (I can't figure out how to include the image here, but if you want it, PM me and I'll send it as an attachment to you). I used images from a website I learned about on the creative community board
http://www.disneyclips.com/
They have clipart for any character you can imagine.
THe kids met their friends at the door and had a hole puncher to "punch" the tickets (this only interested my kids for a few minutes -- but some of the guests took turns doing it as they found it pretty cool). We played a few games -- pin the princess crown on a poster pic of DD3 and pin the light sabor on a poster pic of DS5. We played hot potato to the alladin theme (never had a friend like you). I made a mix of songs from different disney and pixar movies and played name that tune with them. Then made a copy of the mix as a goodie bag favor.
We popped pop corn and had hot dogs & jr. mints -- movie time snacks -- and after all the festivities the kids watched the movie. They all loved it and people are still asking when we're doing our next movie night. Even the younger ones held attention span through most of it.
Although, if you were to do something like that for a crowd where 3 was the oldest, I would choose shorter films -- maybe a jojo and goliath or winnie the pooh or micky cartoons or something -- that would still be related to the disney trip, but would require less attention span.
Also, I love your idea about the pics of them with the characters on the cakes.
Good luck --
