I remember when I saw West Side Story at a local Marriott Theatre, and there were TONS of kids under 10. And, kids were smoking and drinking, people DIED (which, when I was under 10, I was NOT good at handling), and Anita got raped. Personally, I would not take a child under 10 to that, even if they were mature. And really, that's a fairly clean show compared to many.
The thing is, there are many shows that do not neccessarily have anything "bad" in them (unlike Ave Q, RENT, Chicago, Cabaret, etc. b/c they have adult themes), but which you should not bring your child to b/c they cannot neccessarily grasp the plot line or handle the running time.
I think you should hesitate from taking a very young child to Phantom of the Opera, b/c the Phantom will most likely frighten them (and I've seen it happen). Also, once I was sitting behind girls who I would estimate to be around 7 or 8, and they kept asking their mom why the two girls (Christine and Carlotta in Il Muto) were kissing.
And, I remember seeing Aida in NYC- there was a 6-year-old behind me who clearly should not have been there.
Really, I would have to agree that you should only take a child if they are mature enough to handle both the plot/themes of the show and the LENGTH of the show and sitting quietly and respectfully still.
Basically, there really are very few shows that are good to take kids too.
"I mean, how many of us could honestly say that at one time or another he hasn't set fire to some great public building?"