Here is an exercise that comes from Footy4kids- perhaps this will help your kids
"Take off your shoe and sock. Sit on the floor, with both feet on the floor... knees inside your arms...start by kicking the ball out of your hands (like a punt). But, just do it lightly so that the ball only goes up about 2 feet or so.
Make sure that your toe is pointed and that you keep your ankle "locked". Strike the ball with the "laces" part of the foot. If your ball goes up without any spin...or with a little "top" spin... then you are doing it correctly. If the ball comes back towards your face...then try again. Keep doing this over and over...it will develop a good feel for what it is like to hit the ball with your laces. After you have managed that...try hitting the ball twice with the foot. But, make sure that there is still a proper spin...ankle locked...toe pointed. When you get good control it is time to stand up.
Now, do the same thing… kick the ball out of your hands (like a punt) but have the ball only go up in the air about 6-7 feet high. Look for the ball going up with no spin again. Keep your foot in that good position. Your knee needs to be slightly bent as well. Kick the ball straight up in the air. Again, you are just trying to get a good feel for the ball. The ball's spin tells you if you are doing it correctly."
I hope this helps them learn how to kick correctly - Remember these skills take a lot of time to master so encourage your kids to practice on their own.