I've heard very good things about "Slava's Snowshow" playing at the Union Square Theatre. Not exactly a musical (though there is lots of music in it), it's a sort of Russian clownshow appropriate for all ages. Its hard to describe, but I found this:
"Among Slava Polunin's many visual feats is one in which he boards a train and then becomes the train, with smoke billowing out of his top hat. In another, he gets stuck in a web of unspun cotton and clambers over the seats in the auditorium, taking the web with him, until it envelopes much of the audience. In the show's most talked-about scene, he tears a letter into pieces, the bits turn into snow flakes, the flakes become a blizzard and gradually the entire auditorium becomes a raging, whirling, blinding storm that engulfs the audience knee-deep in snow."
Everyone I know who's seen it, loves it (tickets are around $60).
And speaking of clowns, there's a show at The New Victory Theatre (a great space devoted to family-frndly entertainment -- at affordable prices; tix are never higher than $50) called "Aga-Boom" from an alumnus of Crque du Soleil that looks pretty good.
And just to complete the clown theme of this post, The Big Apple Circus is in town through January at Lincoln Center's Damrosch Park.
I've also heard good things about "Children's Letters to God," a musical with a cast of 5 ages 10 to 15, playing at the Lambs Theatre on 44th (tix are $25 to $35).
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
[http://margochanning.broadwayworld.com/]
"The Devil Be Hittin' Me" -- Whitney