Why not use weights? It certainly no more dangerous than Rugby. I am frankly unconvinced by the "lifting stunts growth" or "interfere's with bone development" arguments - they both seem to up there with "lifting makes you musclebound". If the little guy's want to give it a go they'll find a way by picking up each other or some other random object.
I'm not suggesting using massive weights start light work on technique, If you teach them properly, from an early age, you're setting them up for later when they can start going heavy. My youngest (6 yr old) regular comes out to my shed with me. He has a pair of 5lb dumbbells put aside for him to use. He can also deadlift and has managed 22.5kg on an olympic bar and the 16 and 24 kg kettlebells. I don't push him but I let him try stuff if he wants too. Resistance bands are good too (and fun all my kids like to pull with them)
I don't know anyone who has damaged themselves from lifting at an early age (and I know a couple of people who started pretty young). By contrast I know two people with spinal injuries who are wheelchair bound courtesy of rugby (one of whom was only 15 at the time of his accident). I know which sport I'm more comfortable with for my children.