J.I.
We've taken classes at both Gymboree in Frisco and The Little Gym in Plano and McKinney. I highly recommend both. We did Gymboree's sibling class when my boys were 1 and almost 3. Then we switched to The Little Gym so my 3yo could take classes on his own.
Gymboree was the only place that had sibling classes. The other nice thing about Gymboree is that you can attend as many classes per week as you like. Most of the other places will charge more for more than one class per week.
We've been with The Little Gym since June 2005 and have been very happy with it too, although I don't have any experience with their Mommy & Me classes. My oldest has taken gymnastics, sports skills and karate there and my youngest will be starting gymnastics there in January.