If a 12 year old child lifts weights, is it very unhealthy?

No. If the child is lifting the weights properly and not over-doing it, then lifting weights can in fact be healthy as a form of exercise. It is very important that he/she knows how to lift with proper form, and how to not lift too heavy a weight given their abilities. Definitely, make sure the child is supervised by an appropriately trained instructor for his/her age.
No. If the child is lifting the weights properly and not over-doing it, then lifting weights can in fact be healthy as a form of exercise. It is very important that he/she knows how to lift with proper form, and how to not lift too heavy a weight given their abilities. Definitely, make sure the child is supervised by an appropriately trained instructor for his/her age.
No. If the child is lifting the weights properly and not over-doing it, then lifting weights can in fact be healthy as a form of exercise. It is very important that he/she knows how to lift with proper form, and how to not lift too heavy a weight given their abilities. Definitely, make sure the child is supervised by an appropriately trained instructor for his/her age.
No. If the child is lifting the weights properly and not over-doing it, then lifting weights can in fact be healthy as a form of exercise. It is very important that he/she knows how to lift with proper form, and how to not lift too heavy a weight given their abilities. Definitely, make sure there is trained supervision.
No. I'd encourage it if Junior is properly supervised. There used to be a concern that the strain on the bones would damage the growth plates. This never made sense and I'm not aware of any reason to think it's true. Power-lifting is for adults.