Hi S.!
Why not do a make-believe theme? The boys can get into it as well, especially at such a young age. Your daughter can be Cinderella still, but the boys will have a wider option: a prince, pinoccio, wizard, Winnie the Pooh, Shrek, etc. You can mix up the toys (if you're doing goodie bags). Little wands or fuzzy pencils for the girls, plastic bugs for the boys, a box of crayons for everyone (the Dollar Store is great for this stuff: you can get a 6 pack of bouncy balls, a bag of bugs, 4 8-count boxes of crayons and 10 goodie bags. If your kids are anything like mine, the stuff will get played with once before it's left behind and you throw it away.)
Anyway, have fun and happy birthday to your little girl! Oh, and speaking from experience (although, you probably know already) I wouldn't have a party for more than 2 hours, especially where you can't send them outside to play~all my kids have winter birthdays~ugh. Even the 2 hours feels like 6. Have fun!