Younger players tend to concentrate on hitting every ball tossed and mechanics may break down if used a lot. Just an enhancement of the traditional soft toss drill. I find that, especially with young hitters, they will get on information overload. Sometimes you have to force them to use what they have learned...without thinking about it.
Merely put two or even three "tossers" into the mix when doing the soft toss drill. Time the tosses so that the batter has only enough time to pull the hands back, reset their balance and swing. I find that this gives them a true feel for the swing....without getting too bogged down with information.