"Mommy and Daddy are having private play time" works in my house. It did when they were little and it certainly does now that they're older.![]()
About that birds and the bees talk, sooner is better than later. Though age appropriate discussions are suggested.
Books like "Where did I come from?" with illustrations,
or PBS movies like "The Miracle of Life" are great tools.
If you watch or read with them, you have your chance to gage their reactions, ask and answer questions and interject your own philosophy.
Good luck!