South Brunswick, N.J. — How do kids behave when there are no grown-ups around? Donette Mabes of South Brunswick, New Jersey, says you never really know.

"Because you're not watching them at that moment, and at that time," she said.

She had always just assumed her son was good -- until recently, when 13-year-old Gavin Mabes got caught on tape showing his true character.

Gavin and some middle school friends had just arrived at a skate park. The park was empty except for little Carter Bruynell, who was there with his mother celebrating his fifth birthday.

Carter is on the autism spectrum. Big groups of older kids can make him nervous, so his mom, Kristen, was fully prepared to get him out of there. But she wasn't prepared for what happened next.

"I don't know, they really just shocked me. It was unlike any experience I think I've ever had," she said.

You know how middle school kids sometimes operate like they're in a pack? Well, that's pretty much what happened here. Gavin led the way and the others followed. The only surprise was that Gavin didn't start trouble. He started a friendship.

"Gavin is just going around with him and making him feel special. And the rest of his friends kind of followed suit and then started singing Happy Birthday to him," Kristen said. "That really blew me away, 'cause you just want to see the kindness in the world. And I wanted Carter to have a good birthday."

A group of middle school students befriended a young boy with autism at a New Jersey skate park. CBS News

It was such a great birthday, and such a kind deed, even the local police department responded. They decided to throw them all a pizza party.

But here's the best part: Since their first meeting, Gavin and the middle schoolers have continued to go out of their way to play with Carter.

"He was just so happy and he made us all happy," one said.

As for the moms, this was a moment of parenting utopia, where the only thing better than seeing your kid treated kindly is knowing your kid is treating others kindly, even when you're not watching.

"I was just so proud of him. He's good," Donette said.

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