Corrected entry: Harry and Marv could easily see that Kevin was taking their picture with an instant camera. But later Harry asks Kevin to give them the camera, even though the pictures he wants wouldn't be in it.
Correction: He'd likely have asked for the pictures as well if Kevin had given up the camera. When he said "the camera," perhaps he meant with the pictures as well.
Corrected entry: Why didn't Kevin just throw the camera down to Harry and Marv? He had all the pictures of them robbing the toy store, he couldn't lose anything, and the two thieves would then stop chasing him, as Harry said.
Correction: This is, at best, a question of character motivation. Perhaps Kevin thought that the two thieves (who he knows to be repeat criminals) were lying and would continue to chase him after he gave up the camera. Or maybe he thought that they were seeking revenge for what he did to them last time, so giving up the camera would not matter. Also, there was no way that Kevin was going to let them get away with the money, so even if they HAD kept their word, they would've been long gone by the time Kevin showed the photos to the police. One way or another, this cannot possibly be considered a movie mistake.
Correction: Harry had no intention of letting Kevin go, and Kevin was not stupid and knew this. Even if he believed Harry, he had already been to the trouble of setting up the traps and was not just going to let them leave with the kids' hospital money with the possibility of them getting away with it. It was his "good deed" to save it.
Correction: True. Very true. But remember how stupid these two characters are. They aren't exactly the cream of the crop.