A child will be internally motivated to play with musical instruments and any parent who went out and purchased a small keyboard for there toddler, or one of those little drum kits, flutes, tambourines etc, will see that you don't need to motivate a child to enjoy a musical instrument.
Why does this motivation stop?
Parncutt and McPherson state 'Any effort on the part of a parent to pressurise development will tend to destroy the child sense of motivation' (2002)
We all want our children to be the best at what they can be and can swamp this internal drive and motivation. Once your child has developed a fun, musical sense of motivation, harness and monitor this but take care not to swamp with invasive motivation.
Geldard, E., McPherson, G. and Parncutt, R. (2002). The science and psychology of music performance. 1st ed. New York: Oxford University Press, pp.10,11.