We do tumble tots here in Australia. It's often called kindergym here, but for some reason our closest 'gym' is calls it tumble tots.
I love it! Routine is generally circle time, with songs and actions (star jumps, rocket ships) then there are a variety of activities (usually each week is themed = rolling and jumping, then crawling and swinging). At the end we have circle time again and a large parachute the kids lie under and 'tickle' with their hands. So cute.
I see it as DD's time to play and have fun and it works a treat for us!