Honestly, considering the fact they live by themselves in the huge building and are in a pretty peaceful Korean residential area, it's not that bad in this story.
I mean they left them alone once before didn’t they? When they went on their movie date night? They actually seem pretty responsible for kids and the older one could prolly take care of the younger one for a few hours. And I’m sure they were pretty used to being left alone before jaewoo (idk if that’s how you spell his name) became nice and started taking care of them while their brother was at work.
They used to stay alone when hyeonjin was at work, before the current jaewoo checked on them constantly, so i guess they’re independent enough to stay by themselves for a few hours
Did they just leave the 2 kids? Alone? At night?