I'd let her change instruments, take a break entirely from music lessons for a while, or just attend music lessons as long as she continues to make progress/enjoy herself. Six is really young, and people with talent are able to progress quickly on an instrument they love, so there's no hard and fast rule that early = achievement. I don't think there is any problem *not* obliging practise at her age. I'll bet if you did a survey of the musicians on this board, respondents would show they could achieve multiple levels of progress in a year on minimal practise.
To me, music is such an intimate form of self-expression. It shouldn't be firmly dictated. Choice of instrument is really an extension of one's own voice. I think you're wise to consider expanding options beyond piano.
With my DS, I open up the piano and let him have at it whenever he wants. Or, he'll sing a song that he's made up in his head, and I'll translate it into a melody and harmony parts. Or, while we're reading books, we create our own rhythm for reading. He loves seeing the music in action and being able to change it at a moment's notice to suit his mood or play. That, to me, is what music is ultimately about; telling a story or elaborating a feeling!
Maybe this is just me, but playing others' music is only satisfying to a certain point because you're so constrained in terms of creative expression. I have favourite composers, but their music is really just a point of departure for my own composition. The real "release" I get from music is in the experimentation, the free play. Is your DD given an opportunity to compose in her lessons? Is the style of music she's playing consonant with her style? Is she allowed to experiment in her lessons and explore fusion, too?
