Regular home practice is essential for success in playing any instrument. The amount of practice time and (and parent involvement) depends on your childs age and level. My minimum recommendations are:
6 year olds: 10-20 min (3-5 days a week),
7-9 year olds: 20-30 min (5 days a week),
10-12 year olds 30-40 min (5 days a week).
12 and up: 30 min - 1hr (5 days a week)