I struggled with this too when we first had kids. Keeping it to the bedroom only didn’t work for us so we had to figure out ways to make it work when they were around.
Certain things were easy but we also incorporated words and phrases that were just between the two of us. Warning signs and looks always work and a numbering system that have predetermined consequences to be handed out later can be used as well.
I'd love to hear more about your system sub...sounds like you have made it work. We have teenagers so we would have to be very careful. They've already seen the under the mattress bindings. They are not generally allowed to be in our bedroom...
Mine are older now (teens) we used signs and symbols too. Like a hair brush left on a counter top was "that was wrong, spanking later" it was a way of saying things without words immediately. I think as long as you appear to be in a loving and happy relationship a child will accept things like a partner sitting on the floor etc ....I think its how its present more so than what you are doing.
We have a middle schooler and it is so hard to do 24/7. But we are slowly finding our way and because of the way kiddo2 is we feel like some of it can be more overt and she would have no issues with it. At this point we don't hide a lot of the conversation we have about it (within acceptable reason though). However the sadomasochism I am against sharing with her at this stage in her life. It has become a hard limit for me to allow anything that would be noisy when she is around. Which is actually much harder on me than it is on him.