I checked out Mini Maestros on the net, but the classes seem a little bit more structured and serious than I would like, and I have heard the same.
That's odd, yes it is structured, but they did not come across as serious to me, but rather fun when I was taking Maggie. She definitely enjoyed it, but was much more interested in dancing than in instruments. May be it is very dependent on the individual instructor how good the classes are. I found Hey Dee Ho to be less fun, but then again I did not warm to the instructor.