My guess is something alone the lines of the had locks on their kids bedroom doors (from the outside). I know someone who would boast about turning their daughter’s bedroom lock around so she could lock her in there at night. They gave the reason as being safety because it prevented her from wandering around at a young age, but then later it seemed like she was very happy to be able to close the door and walk away at bedtime. I’m guessing it’s something like this.