Okay, so there was this movie I saw as a kid and I completely forgot the title. Okay, so this dad has to take care of his daughter because something happened to the mom when she was born, and the grandfather helped up till this point, but he can't anymore. The little girl always forgets to flush the tolit, and at the end they do a song from Sweeney Todd, when all the other families did Kats. Oh, and at one point the little girl did the "I'll show you mine if you show me yours" thing, and got into trouble for it.

Jersey Girl. Here's a snippet that matches your description:

....One day he picks her up from school in the city street sweeper and she announces that there will be a parent/student musical review and that she has volunteered them to sing a song from the Broadway show “Cats”. Ollie balks at this and tells her “no”. She begs and he promises to take her into the city soon to see a Broadway show to make up for it.
Gertie chooses for them to see the very bloody musical “Sweeney Todd” while they are in New York and the two have a great day seeing the play, talking about her mother, and taking a carriage ride through Central Park...

Jersey Girl (2004) - IMDb (theres another movie with the same name from 1992, doesn't look like a remake, think this is the one you want... )