P.H.
My daughters started piano lessons around first grade.
The piano will help them understand music theory for other instruments. Knowledge of the piano is helpful for a BA degree in Music.
My oldest daughter graduated out of CSU with a BA in Music Education and is now a music teacher. She played a beautiful sounding clarinet. The youngest daughter is at the San Francisco Conservatory of Music and is a flute specialist. While they were in college they WERE asked about their knowledge of the piano.
Make it fun.