I have thrown 2 parties at the Wilmette Park District gymnastics facility. For my twin boys' 5 yr and 6 yr birthday parties. Both were HUGE hits! The staff there are so great- very professional- adults, not teenagers. They have a large expansive facility. They do real gymnastics with the kids, not the My Gym cutesy stuff!
Prices are very reasonable- it's $140 for 0-9 kids for one hr of gymnastics and 1 hr in a party room. $60 for each additional 9 kids (so $200 for 0-18 kids, $260 for 0-27 kids, etc). For an additional $90 (I think) you can rent the large party room- which I highly recommend- we did it this year for 27 kids and the room was awesome! The kids have always had a great time, and what I like is that it is structured and interactive, the kids actually are engaged with each other constructively- whereas with the pump it up places the kids just run crazy with no real engagement or structure!