My wife and I decided to use a secret keyword when we want to be intimate so that our 5-year-old son wouldn't find out.

We chose the word "typewriter."
One night I told my daughter: “Go and tell mom that dad needs to write a letter.”
She went to see her mom and came back saying:
"Mom says you can't right now, there's a red ribbon on the typewriter."
A few days later, my wife told her: “Go and tell dad that he can now write that letter.”
She came to me, then ran to her mom and said: dad said "it doesn't matter," he already wrote the letter by hand and it only took a minute, and he told me not to tell you!