need helpful advice

OK my wife and I really want to see her boyfriend more. Its so hard with our kids now that they are older. My kids are never going to know and I accept this, they go with their mom every other weekend this is when we want to hook up with her boyfriend. But we still have my wife's 2 kids who are 14 and 7. My stepdaughter is a great kid and we share a lot of secrets. She is becoming sexual and I've been able to talk to her when her mom can't. I've thought of breaking the news to her that her mom and I enjoy sex with other people as well and that we would like to see these people on a more regular basis. I'm kind of scared and don't know if I should say anything. We want her boyfriend who is discreet to come over and hang out as friends and then bang my wife in the privacy of our bedroom. Problem is kids might hear her scream and moan from him and we need to be able to explain. Should I tell her or not that's the question? Advice?