Will lifting weights when you're a child hurt your joints?

Weights and kids. Lifting weight when your growth plates are open (immature) can damage the growth plate. Since most weightlifting is not particularly beneficial until after puberty, most doctors would recommend focusing on excercises that improve coordination in a young athlete, rather than those that involve heavy weights which can create shear and compression across an open growth plate.
Yes. Strength training is ok an any age under the guidance of qualified trainers. The growth plates in the long bones are not as strong as ligaments, so little johnie playing conan will fracture a growth plate before he sprains a ligament. The aap recomends a kid be at stage 5 (end) of puberty before involvement in power lifting or other such events. If shaving daily, stinky & <1in from final height, ok.