@D-Glad it's not a bug but it's the game's way of not leaving a household with an underage child. It's classified as being taken by a Social Worker but effectively the household is culled as the only remaining inhabitant is underage. It's the same if a pet is left behind. If it happens in gameplay there is usually a message.
If you have the option to go back to a previous save if you are on PC/Mac then that's the only option I can think of. I play on Xbox Series X and when this happened in my game recently with pets I had a backup game from an earlier point in the day. It's always a good idea to keep a backup incase of things like this. You can also exit without saving in some cases if it will allow you, but that's if it's your active household.
If you are playing with Neighbourhood Stories enabled you can also turn it off or turn off accidental deaths as I'm assuming that's how this happened.
https://www.ea.com/games/the-sims/the-sims-4/news/neighborhood-stories-system
You could look in your household menus for them, try searching by Currently Not in World. But it's doubtful given the household child age status, still worth a look though. See if the child is in the Sims relationships and invite them over but if they are gone then it's probably too late. I've actually seen a player have this happen when a parent was remaining but in a different household, it's quite heartbreaking.
If you are playing on PC with mods I'm not sure if there would be a mod to help.
