At that age, not really sure if I am good with just letting guys throw right down the middle of the plate. Maybe divide location into thirds: rather than setting up with the targets on the corners, setting up middle-away, middle, and middle-in. While throwing down the pipe may have a positive effect on the bottom line in terms of strikes thrown, reducing walks and HBP, maybe even runs (depending on the skill of their comp), these kids are still developing and I would still want them to get some feel for location and learning how to pitch. You still want to stress setting a good target and the pitchers focusing on hitting the glove.
Are these kids just not great players regardless of age? Are their mechanics completely butchered? Are they trying to do too much? Are they scared to be in the zone and nitpick because they are worried about getting hit hard?
Why is a summer league team practicing only once a week? Even in that one practice, I am sure a flat ground session between starts would help.
Just my opinion though