Praise. Children love to be praised. Praise them for finding the first note, for sitting up straight, for a good hand position or tell them you really like the piece they are playing. If you fuss at them they will tune you out. Sit and listen to them or dance around the room and act happy while they play. When I was a young student, I decided one summer to practice while my family was having lunch and my mother let me do it and eat after everyone was finished. I think I just wanted everyone to listen to me play the piano using practice as an excuse. Also, ask your child to play for neighbors or friends when they come to visit. Even if it's not the best playing, your friends will show their appreciation.