Better plan for the water route in case you go that one:
Marbles. In the freezer.
You dump them under the sheets and no matter which way they try to roll and move, the gravity made sure the marbles stay right next to their skin.
I had to get clever, my teenager just said "thanks for the free shower." and went back to sleep, no matter the temperature. ^.^
I'm lucky though, my kids get up on their own now out of habit. I have a temper that would scare the Devil himself. (Raised in Texas for 20 years, that'll do it to you... )
Either way, good luck!!! I hope you find a stress free way for the both of you!