Maybe the dad didn't realize the mom had put the money in. Maybe he thought you had to press a button first. Maybe he thought he hadn't latched the door shut all the way. Maybe he'd never been to the laundromat before and didn't realize the doors locked.
But if it makes you feel better to think of them as monsters rather than idiots, have at. You gotta do what you gotta do to shake an image like that.
Awful lot of maybes in there, to be all chance, you must admit.
But even supposing every single "maybe" is true, we're still left with one undeniable fact. Douchebags thought itwould be funny to close a kid in a washing machine.
So they're guilty about being dumb about what would happen next AND guilty of cruel douchebaggery. I don't think they were trying to kill the kid. But I do think they didn't care much if they scared the shit ouf of the kid in the name of a joke.