Why does Kate keep locking the BR door before sleep?

I noticed that as well and the only possible explanation would be that the maid walks around the house as she pleases and at the beginning of the movie she just walks in casually and announces that there's a lot of traffic. it might be a sign of their position, they don't have to worry about children but about the maid walking in on them. my guess.

I, too, wondered about that. In addition, to controlling the maid, I thought they locked their bedroom because they were well known attorneys. The locked door gave them a bit of security. A semi panic room...

:)