I'm sure there are a ton of technical ways, but the way I taught my little team of 7-10 year olds to literally underhand throw the ball over the net, kinda like a bowling or softball pitch move. That way, they know how much force/speed it will take to get the ball over the net. Do this super simple drill enough times until they get tired of doing it, take a break and do it again. Note that it may be a little difficult to get their arms back when holding the ball, but that's where the speed comes in. They'll learn to not hit themselves, not just throw it up in the air and hopefully even target.

Good luck!