A safe, easy way to introduce kids to shooting rifles is to take them to the nearest Cabelas or BassPro shop. Usually both of these stores will have laser ranges set up for the kids. Call ahead just to to make sure though.
The rifles all shoot out “lasers” of some sort. As long as the kids are not pointing the rifles at each others eyes, there should be no problem at all. Parents should be there to supervise the kids and give them instructions.
The guns all have iron sights, just like a BB gun. If the kids hit certain targets, usually something will move to indicate that they have hit the target. For example, a deer might pop his head out of the bushes.