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Gilbert Arenas says he's so hard on Nick Young's 4-year-old son ... because he doesn't want the kid to be a "vagina" when he grows up.

Gilbert has been posting videos with 4-year-old Nick Young Jr. -- doing things like throwing his scooter across the yard to toughen him up. He also mocked the kid for crying when he got hit with a basketball.

So, when we saw Gilbert at Skateland over the weekend for Dorell Wright's '90s-themed charity event -- we asked if he thinks he's going TOO hard on the youngster.

"Hell naw! That's how you're supposed to treat boys," Arenas said ... "can't treat boys like little girls."

"I'm worse off the camera to him. He used to be a vagina, now he's good."