Posture and balance is the foundation of a good, repeatable delivery. Watch their heads for inapropriate movement left/right, back towards 2B or up/down (recognizing that a certain amount of "drop" over the course of the stride is normal). Try to get your pitchers to eliminate such unwanted movement.
Pay particular attention to their starting position on the rubber. If they start in the stretch position, make sure their feet are no more than armpit width apart with back foot toe aligned with front foot arch. Also consider putting some bend in the knees and waist (i.e. put them in an athletic position) - how much will depend on each pitcher. Tall skinny kids may need to be scrunched down more than others to lower their center of gravity and help them stabilize their posture as they move down the hill.
Also watch their glove. Dropping it, pulling it back or flying open (swinging out to the side) can affect posture (as well as other things).
These things will help the kids start to develop some consistency which should translate into consistency at the plate.