We have used this very old and simple drill to get the young pitcher to deliver more consistently controled pitches.
Find the spot that you want to get the pitcher's lead foot in on each throw. Nail a cheap (thin) paper plate to the ground in this spot.
As the pitcher becomes comfortable hitting this target you can make a "box" from very thin strips of wood. "no more than 1 inch deep" Be sure that the box is a little bigger than the pitcher's foot. Use this in the same way as the plate.