If the number isn't too great, there's a cooking studio in town that caters to birthday parties. They have three large work tables, with each table accomodating 10 people, so 30 kids shouldn't be a problem.
So your son and friends will be having a bake-out as well as being able to take home the cookies and cakes they've baked!
Details found here:
www.soez-cooking.com
Well I thought of this place because I think kids love playing around with dough, and they get to exercise a bit of creativity, and most of all they have fun!
Cheers