Sing For Your Supper is a good one - gets pretty intricate and crazy with the jazzy harmonies and stuff, but I love it...
Never Never Land as it is in the show (if only the music were available...le sigh) from Piece is beautiful
I Want It All from Baby (might not be age appropriate though, not sure)
At The Ballet - oldie but goodie from A Chorus Line
There's Gotta Be Something Better Than This and/or Baby Dream Your Dream from Sweet Charity
I Wish I May and/or Make Him Mine from Witches of Eastwick
Somebody's Eyes from Footloose
Lotta Locomotion from Starlight Express
I've seen people do By My Side as a trio - just splitting up solos and stuff (usually only 2 people singing at one time until the ending and you put a 3rd person on one of the choral harmonies)
I gots millions more, PM me if you need more suggestions, lol
Hope that helps!
If I show you the darkness I hold inside, will you bring me to light?