This is a drill that always helps me.
Get one person to shoot the basketball, have two or more in a rebounding position. When the shooter shoots the ball, see who can get the rebound.
You would need 3 or more
I never make the kid run who gets the most rebounds, it always makes everyone try harder.
It can work at any age, just try to do it around 10 times every practice.
This basketball drill is licensed under a Creative Commons License. This drill is from Pål Degerstrøm at www.degerstrom.com/basketball.