Kids should never be scolded, punished, etc. for experimenting with and exploring their bodies, on the contrary, they should be encouraged to do so!! The concept of it being "sinful", comes mainly from the story of Onan. Sin is a man-made concept, there is no sin in nature!!!
And most mistake "the sin of Onan" to masturbation. It wasn't that. It was failure to try to provide a male offspring on behalf of his late brother to his brother's widowed wife, who was still childless. Back then, there was no "safety net" for the elderly widows, so for them to be cared for, required the eldest son to take his mother in in her old age. (Thus why Jesus, as He was dying, assigned care for His mother Mary over to John, son of Zebedee, one of His disciples.) That was their version of "Social Security."
As an interesting aside, some members of the Jewish High Court, the Sanhedrin, question Jesus about this subject, with the hypothetical eldest brother marrying, dying childless, #2 marrying her and also dying childless, and so on till #7. "So, Jesus, whose wife will she be at the resurrection?" Now the irony of the thing was, the people who were challenging Him were "Saducees," a sect of Judaism which denied the resurrection and the existence of the spiritual world. Anywho. too long an answer.
If the Bible is the standard in your life, then masturbation is not sinful.
Onanism is another word for masturbation. But Onan didn't get into trouble for masturbating.
Onan's older brother died without producing a son. It fell upon his younger brother to produce a son for him. However, Onan wasn't married and didn't have any sons of his own -- and didn't want his own first-born son to legally belong to his older (and dead) brother. On the other hand, Onan had to trouble with fucking Tamar. However, he'd pull out and shoot his load on the ground.
In the end, Tamar tricked Judah (Onan's Dad) into fucking her by pretending to be a whore -- and, yeah, she got pregnant and had her son.
And that's the ONLY reference in the Bible to masturbation and it clearly has nothing whatever to do with masturbation.
And people think the Bible is boring.