By far, the best place to host a kid's birthday party in the Madison. We've done at least half a dozen parties here as a family. Very nice, helpful and thoughtful staff, the most reasonably priced option in town. And you can bring in whatever you want for food and drink (including beer!) and they don't charge you by the kid for private parties.
Our kid got so sick on the day of her party -- we had to cancel. I can't say enough about how understanding and accommodating the ownership was.
Pro Tip - Legacy is popular for parties. Reserve WAY in advance in you want a weekend slot.
Open gym can be packed, particular during bad weather.