It was a foreign language film, not Korean, some kind of apocalypse where people could no longer go outside. Maybe they were in apartments? The main characters end up having a kid together and fast forward to when the kid is like 10 and the kid CAN go outside and a group of other kids show up and the kid leaves with them while the parents have to stay inside. Watched it a few years ago, definitely not any older than 10-15 years.