is nigh on impossible?!?
When kids are little, I thought sex was difficult, fear of them waking wandering in etc. But actually by 8pm they are tucked up in their beds, a tornado could rip through the house and they wouldn't wake up, and god forbid they walk in on anything, daddy tickling mummy is accepted as a fantastic game, and no emotional scarring.
Teenagers - they don't go to bed/sleep until you've gone past knackered yourself, they know full well what sex is, so you want to ensure there is no noise, no nothing that will give them the impression their parents are at it in the next room.
You wait for them to go to all night parties just so you can have a sexlife!!!