One of my sims kids refuses to stay in bed. No mods installed. Other kids in house will stay in bed.

Me too
I have one kid that always wakes up few seconds after going to bed, and I need to put it to sleep 2 or 3 times to finally sleep

There is a current issue thread about this in Answers HQ:
https://answers.ea.com/t5/Bug-Reports/CURRENT-ISSUE-Sims-won-t-stay-in-bed/td-p/6422023/page/29

Some things that were suggested were:
- Making sure there is nothing making sound in the room and disturbing your son such as a stereo.
- Replacing the bed.
- Exiting to the main menu and reloading the game.

This sounds annoying, I hope it gets solved for you. :smile: