This is factually incorrect.

Perhaps reading TFA would be helpful.

Prior to the onset of puberty, weights should be fairly light (as the capacity for the body to put on muscle is limited) and the focus should be on technique. 35kg is pretty reasonable. More (as in pretty much everyone) people should lift weights, and that kind of knee jerk reaction is harmful.

• https://pmc.ncbi.nlm.nih.gov/articles/PMC5532191/
• https://pubmed.ncbi.nlm.nih.gov/18461111/
• https://examine.com/articles/is-weight-lifting-bad-for-kids/
• https://www.stanfordchildrens.org/en/topic/default?id=strength-training-no-longer-off-limits-to-kids-1-1187
• https://www.mayoclinic.org/healthy-lifestyle/tween-and-teen-health/in-depth/strength-training/art-20047758

Yes, they should climb trees and wrestle each other too. Both those activities have a higher incidence of bone injury though.

Is it? This seems to say otherwise - I'd be interested to read studies to the contrary, though.