My son is 4 . he has been doing taekwoondo for 5 m. concerned if running with weigh ball is harmful for his joints/muscles. is it to early ?

Individual results: If he is sore afterwards, he is doing too much. Make him stop. If he has not soreness the next day, it is ok.

Exercise: Children are best allowed to exercise to their own ability. I think it would be difficult to compel a preschooler to run with a weight too heavy for him/her. If your child seems to be able to tolerate it without apparent pain or exhaustion, or coercion, I would allow him/her to continue.