Really desperate for this movie!

Its about some kids (I think maybe 2) go to this big mansion type house. There, they learn all about musical instruments, such as woodwind, bass, and they learn about a quartet

There is a tiny man who I think lived in a box who was from timbukto, who played many instruments, like the harmonica. He eventually stole an instrument from the house

If anyone can name this I will be the happiest person ever

Thanks