DD started at 3 and did two years of Ballet. Interestingly if anything it did teach her good posture. People comment even now that she has a dancers posture, and she was only 3 and 4 when she did it. She wishes now she had continued, but at 4 she wanted to give it up. I feel bad that I let her too...

There were a few shy kids, but they really came into their own over a few sessions. If the teachers are good it shouldn't really matter. Goodluck!