Telling children how sex works is important. You can do this when they ask about it or when they reach some level of sophistication that will let them understand the explanation you're ready to give. Telling anyone - especially your child - that you just had sex on the couch is a poor choice (outside of some plausible dynamics that consenting unrelated adults could set up). It's none of their business, and a psychologically typical child won't want it to be their business or will be embarrassed to have so wanted when they get older.