Hi there
I'm in a similar situation for my three year old's upcoming birthday.
My plan is to get one of those large coloured parachutes (used at gymbaroo), with a few toys and balls etc and we can play some games eg. jumping joeys on the bed and remove a joey for each time... that's all I have so far but I've seen loads of sites on the web for games. I've been told you can buy the parachutes online for ~$20 so if that's true it will be much cheaper than professional enteratainment!
I figure it should be fun for the range of ages and maybe even some of the parents can boost numbers if needed. I'm just hoping they wont get bored after 5 minutes!
Anyway.. just an idea for you...

ps. don't forget to get those multi-pack bubbles.. always a hit at birthdays.