There is an earlier post about keeping your “hands inside the ball”.
Here is a great drill to keep your kids from casting their hands away from their body and elongating the swing.
The other drill is facing the net. Hold the bat at your belt buckle extending toward the net so that you are the bat length away. Swing normally without hitting the net. This forces you to bring your hands through the zone first. The natural motion of your wrists and bat will “throw” the bat through the zone creating bat speed.