I find especially the younger ones often ski backseat also,
you can work on a little stance and parallel at the same time as making it fun by jumping between turns. This requires a balanced stance and feet underneath (i.e. skis parallel).
for up and down motion I like the drill: Tall as a house, small as a mouse
for this, extend your arms fully upwards and straighten your legs, emphasize being as tall as possible - "tall as a house". During the turn make yourself "as small as a mouse", knees bent etc...
don't sorry too much on the "right" amount of up/down motion, it's important for them to feel it for themselves and find the balance themselves.
There's plenty of other drills too, do you have your CSIA handbook? look through for examples, and contact me if you want more/ more questions
hope this helps