Imagery works wonders with children and it's fun!
Instead of "curve your fingers" try "pretend you have a golf ball under your knuckles"
Instead of "keep your elbow up" try "pretend you have a quarter on your elbow, try to keep it on there while you play this piece"
Instead of "play loud" try "play like a lion"
Instead of "play staccato" try "play like your keys are hot coals"
Sometimes I will actually try the visual to help them with the image.
You get the idea!