E.V.
First of all I am glad your kids are enjoying sports! I am a gymnastics coach as well as junior high teacher and I can tell you many kids are not active enough. I am concerned that you feel they are ready for light weight lifting. Why? For more muscle tone? Junior high kids don't need muscle loading unless the sport they are in demands it, and then the coach will usually take care of it. If you simply feel they should gain muscle you need to look at your reasons. You are treading on dangerous territory. It's more about what THEY think and your judgement can send them into all sorts of difficult thoughts. If they have come to you and expressed interest, I would send them back to their coaches and have them handle it. If it's solely you noticing, I would keep it to myself. Kids that age have enough to worry about. Usually in high school PE they get their first lessons on weight equipment for health. This is appropriate to match their development. They need to be more fully grown before stressing any growth plates. Perhaps add in some fun activities that will work different muscles... rock climbing for the runner and roller blading for the wrestler. Mix it up for interest and health!