I played some form of sport everyday as a kid; football, cricket, rounders/stool ball, swimming, running, cycling and tennis mainly. Didn’t do me any harm until I got to my late teens when I start getting shin splints. IMO I put that down to incorrectly (basically not being interested in) warming up and warming down.
I know friends with sporty kids and they don’t yet seem to have any issues. I know the body is in a major growing phase at that age so I suspect while some kids have the physiology to deal with it others just don’t. No doubt there’s a tad more science to it that that.