Whether the dad was wrong to call the cops isn't the issue. She handled the situation wrong. If she didn't want to give the ball back to the kids, she could tell them that she'd give it back to the dad, that way the two of them could have a talk about it, LIKE ADULTS.
I'm of two minds about this kind of thing. On one hand I don't think it should be illegal. I think the law is only set up to benefit a few industries that would be greatly harmed by marijuana being legalized. On the other hand, its still illegal, and you could get caught like this kid and have your life ruined. Best not to risk it.