B.B.
I don't know if this is something you're interested in, but the gym I take my daughter too has a "birthday party" program: http://www.sportastiks.com/Birthday_Parties.html
I've seen kids as young as 1 1/2 to 2 taking classes and my daughter is 5, and LOVES it. I thought this looked like a fun idea.
For what it's worth...and good luck!