Not much should change based on how hard a guy is throwing. I don't recommend changing where a hitter stands in the box. If he can cover the whole plate and is comfortable in a certain position in the box, keep him there no matter who is throwing.
The only thing I like to remind a hitter when he's facing a guy with a little more zip is to make sure he has his front foot down earlier. A swing can't start until your front foot is down and a lot of times kids struggle against a fast pitcher because they don't have their front foot down in time. Get your front foot down early, while keeping your weight evenly distrubted, and throw your hands at the ball.
Now, good luck drilling that into 3rd and 4th graders heads...
Maybe you can contact a local batting cage place see if they would provide a reduce rate or even give your players some free time during non-peak hours. Or how about renting a pitching machine?
Or find some older high school kid that has a good arm and see if he wants to do some community service time as an assistant coach.
Or tweet Brandon Phillips and he might bring over Chapman to pitch to the kids. ;)
Like Freak said, at that age body control and mind control can be a bit tough to manage, so some kids might have trouble with timing and balance (and remembering).