Why do people think it is ok for kids to masturbate?

Its natural so OK. Why should anyone think it isn't, provided it is done privately.
Debatable. You would have to define "kids" in a sense, humans masturbate from the time their hands find their genitals, both male and female. Pleasurable sensations evoke continuing the process. Public masturbation is not an appropriate thing in children old enough to comprehend or to be redirected. Otherwise, no one can stop "kids" from this. After a certain age and maturity, they will on their own.