I wouldn't recommend Wicked: it doesn't move too quickly and I don't think my kids could sit through it. It's good: but the ideas are more adult (I don't mean naughty! I mean just in life-lessons).
Keep trying Wizard: any seat in the circles would work, and some may become available.
Shrek also opens shortly: I'm sure the previews must be happening soon.
Otherwise: Mama Mia: great: especially if they know the songs, and Lion King.
What about Legally Blonde? Sure the life-lesson/ideas and some jokes may go over their heads, but it's bright, colourful, fast-paced, and there's a dog. My kids love that stuff. It's also just won loads of awards.
OR: Any of the "big" long-running musicals are a good bet for getting your money's worth, but I'd probably try to fill them in on the stories/plot/songs beforehand: i.e Phantom and Les Mis are amazing re songs and set BUT an 8 year old wouldn't follow much if it wasn't already explained. Good thing with Phantom is that you could show them the film beforehand too, but perhaps some would think it's too grown up?
P.S Don't listen to me too much: we LOVE musicals in our household, and I'm not very good at censorship: my kids know all the words to Sweeney Todd, and the youngest is 7... 