Black and white theme (I went to a great 21st last year, held in a hotel, the birthday girl wore an amazing white dress and her bf a white tux, and everyone else had to be in black dresses/tux. Balloons and table decorations and everything was black and white)
Pink theme? EWveryone has to wear one item of pink (or any other colour)
Superhero party
Pirates
"What did you want to be when you grew up"
Famous couples/Hollywood party
Fantasy characters
Pimps and Prostitutes/Tarts and vicars etc
Think of a letter (maybe your initial) and everyone has to come as something beginning with that letter.
Miss/Mr World pageant party - everyone has to come as a different country (someone could dress up as Statue of Liberty, Viking, geisha girl, russian doll, one of those yodel girls like in Charlies Angels 2, boys could come as a typical French character with onions and a beret etc etc)
Childrens party idea is great - you could always hire a funfair ride or have Coconut Shy/Shoot the duck stalls where people play for drinks or silly prizes etc.