When he was 5 ours found out that I was hurting his mom, which was the bit we'd tried to keep from him. (She had started dressing to show off her bruises, because she felt proud of them and it amused her to freak the girls in the office, but neither of us noticed that it meant the kid seeing them too.) So I had to have a serious talk and explain that some people enjoy hurting or getting hurt as part of making love, but not everyone feels that way and nobody has to do it if they don't want to.
With the ex-slave I tried to be more discreet, mainly because the stuff I was doing to her was so extreme that even a hint of it would have scared a 6yo badly. This meant sending him to play with friends down the road a lot when she was around, which in hindsight wasn't a good idea.
What happened after that meant he learned far more about my affairs than I would have chosen to tell him, and the up side of that is, now that he's 16 I'm sure he has a pretty good idea what we're into without anything said.
I'd imagine he doesn't want to know about it any more than any teenager wants to know about his parents' sex life. But judging by the sort of websites he was visiting before he learned to erase his online history (I'd never heard of BDSM anime games before he led me to them) he at least understands the attraction of bondage and SM, so I'm not worried that he's going to panic if he comes across a cane we forgot to put away.