A thorough discussion of the pros and cons of young kids lifting weights goes far beyond what is possible in this tip. What is key for parents to recognize is that some simple suggestions can let kids lift weights while minimizing the chance of injuries.
First, I would argue that kids who haven’t reached puberty aren’t going to develop a bodybuilder’s physique, and they shouldn’t try. Instead of performing a huge number of different exercises, I would encourage them to do a smaller number of exercises that work the larger muscle groups.
Next, you should teach them proper techniques with each move rather than flailing their arms or legs. Also encourage a higher number of reps with a lower weight they can control rather than a few reps with much heavier weights.
Having a spotter to pick up the weights if they can’t complete a repetition can avoid serious injuries. Having an adult present not only to spot but also teach the correct form with exercises would be ideal.
You can argue the merits of letting young kids lift weights. Regardless, I think these steps can at least allow them to do it more safely.