The thing about many of the shows you suggested is that will kids want to see Curtains, LoveMusik, or The Pirate Queen? Most kids would be more inclined to see Wicked, Mamma Mia, Beauty and the Beast, ect. All kids are not alike but if a kid really would want to see one of those shows, I think they would make an effort to learn as much as they can about them and be able to tell you whether it would be appropriate or not.
For example, I saw Avenue Q when I was thirteen. Both my father and I really enjoyed it. However, my dad was far more shocked at the show's material than I was, because I was very familar with the cast album while he had no idea what to expect.
I just can't understand why anyone would take a child under ten to see Les Mis. It's not quite G-rated... I can imagine a child wondering about "Lovely Ladies"... Besides, I know people who have seen Les Mis when they were little and were totally turned off to it as teens because they didn't understand it when they saw it and thought it was boring.
When taking a kid to see a show, it's more important to find a show the kid will enjoy than one that's appropriate.
Cosette: Roses are red.
Marius: Violets are blue.
Eponine: You're so in love!
Marius: And so not with you.