PHANTOM.
Looks, it's one of thegreatest showsever, right?
But more to the point, I was 5 when my parents took me to see "Pajama Game" (1955) -- I still remember the 'magic' of it. The dancing, the movement, 'all those people down there.'
If you want to start her out with the best, the costumes, the music, and yes, the wonder of the story -- you really cant go wrong.
And, I'll also second the recommendation about LION KING. It's the first show I take folks to see when they've never seen Broadway in New York. I still believe the opening number would make anyone a fan of theater for life. Lots of laughs, some pathos, beautiful melody, all those puppets!! and a happy ending.
So I was GrandPa, I'd take her to one of those two. And a piece of cheesecake after the show.m