If truly consistently that far off the plate, at some point, coach would have to express concern directly to ump but problem is the coach has the worst angle to see just how far off it is being called so not much credibility. If it is somehow otherwise clear that the zone is stretched way too far, coach can submit a request for ump not to do his games with the reason why.
But this needs to be kept in perspective. JV baseball - zones are typically wide. You don't want to be that coach unless there is a clear and blatant issue. Spend a lot of time instructing on hitting oppo. Teach them to get closer to the plate with two strikes so that they can cover pitches just off the plate. Nothing at all wrong with the knees. It is a strike.