Lincoln had to do a parent/child class until a month before his third birthday, and those were great! He has absolutely LOVED the classes that he's been able to do by himself, though. And he's done so much better with participating than he did when we were with him.
So what are the toddler classes like? Well there is time doing games and exercises led by the teacher, there is free time (in the parent/child classes) to run around and explore the gym with parents, and there are activities that require a group effort. In the "Funny Bugs" class (3-4 years old), they do opening activities together and then split up into stations and work on skills (bar, rings, tumbling, etc.) in groups. For older kids, it's mostly just gymnastics. There is a sports skills class, which I'm thinking about having my three-year-old try out soon, but I'm not sure how it's different exactly.