Boy, they never had anything this cool when I was a kid. If you want a great idea for a kid's birthday consider an 'animation party' at the NFB in downtown Toronto. All the partygoers get to make (and keep) their own plasticine characters and shoot them using stop motion photography. By the end of the party the kids have created their own short animated film. It may sound a little technical, but the whole process is explained and overseen by NFB staff in a way that kids can follow no problem.
First there's the character construction....
You end up with characters that may or may not look something like this...
Then you place the characters on a tiny animation background moving them a little at a time, shooting them frame-by-frame...
You're almost certain to end up with a great (and possible hilarious) animated flick. Of course since these are birthday parties you shouldn't forget the birthday part of the party at the end...
You can get more details and info on how to book by checking out this page at the NFB's site.