1- Keeping their eyes on a level plane. Many kids think if they take a large step, they will gain more power. All it does is put their eye level all over the place. At a young age, kids need to lightly pick up their front foot and land it in basically the same spot. This keeps their head steady.

2- Kids tend to open their front foot. This prevents them from driving pitches on the outer half the opposite way and takes away the ability to generate power by hitting against their front leg and driving with their hips. A good way to prevent this is having them slightly pigeon toed to prevent them from opening all the way up.

3- Sweeping. Instead of throwing their hands at the ball, kids tend to swing way too wide. This takes away power because they can't snap their wrists, it makes the swing too long and slow, and causes them to roll their hands over...which is a huge no-no. A simple drill for this is to put the bottom of the bat on your belly, and the end of your bat on a wall. Then practice swinging without hitting the wall. Simple soft toss is also good for this.

Hope this helps.