You won't go wrong with having a keyboard in the house, but that's just a start. I think the majority of kids start with piano lessons, as I did, but most who become musicians don't remain piano players. The essence of musicianship is learning to play with other people and piano for all its virtues is usually a solo instrument.

When I was a kid something that helped me develop basic musicianship a lot more than the piano lessons was a couple of years of "Orff method" group lessons. This consisted of small groups of children playing simple instruments such as recorders, glockenspiels, drums, triangles etc. and singing--learning the basics of keeping time with others, harmony, arranging, and so on. For some reason the Orff method is not much heard of these days, its all Suzuki "turning your child into a wunderkid by the age of three".

I started my daughter on guitar a couple of years ago, she worked at it for almost a year, then got involved with flute instead. (An instrument with a lot of virtues; its easy to carry around, and can fit into many different musical styles; folk/celtic, classical, jazz and more.)

My so far unproven theory is that starting a youngster on a mandolin with only four strings on it (single-string courses instead of double) should be a great way to start learning tunes without all the tuning hassles, and the finger pressure needed, for 8 strings. And you very seldom find too many mandolin players anywhere.

Willie-O