Some of the greatest musicians of all time got an early start. Aside from a nice hobby, there are studies that show a positive correlation between kids who play instruments and academic achievement. As someone who played piano as a little kid and quit only to take it up again on my own terms a decade later, I suggest having them play an instrument if they want to play it (or you anticipate they will want to play it). Forcing your child to practice every night will likely not engender a love of music.