they didn't want to because they knew they were at fault.
Well we don't know that. If we could have seen the reason why they took the kid, then maybe we could judge. I suppose if they let the kid go that easily then they didn't have a good reason but still.. All they would have had to do was identify themselves and leave. If the guy was doing something illegal or harmful to the kid, or around him for some reason, or a different logical reason (which probably wasn't the case) then they could have just left instead of looking like morons on camera which will get them into even more trouble.
If there was a legit reason for taking the kid then that guy is an starfish.
If there wasn't a legit reason for taking the kid then good for him.
That's pretty much how it goes.