Don't give it a 2nd thought. Plenty of older siblings will be there.
DS is older than 9, 5'1", not thin, and he LOVES Disney Jr. Concert. No one gives him a 2nd glance, and never has anyone minded that he gets up to dance. (He is special needs, but its not 'obvious' at first and still no one cares - i.e., they aren't just being nice to him because he's cognitively disabled.)
It's a very pleasant, casual crowd. I've gone dozens of times (probably a dozen just in 2014).
FWIW, there are benches in the far back of the room where grandparents and pre-teen siblings (my other DS!) can go and sit away from the show if they wish. If your 9 year old thinks its too childish for him/her, s/he can go sit there with a phone (we put ours on mute and DS plays games).