My partners and I have seven children in all. With the younger kids our relationship is so far off their radar that we don't worry too much. We show affection in front of them. Sitting together on the couch, holding hands, even sharing a bed at night. So long as their needs are being met they are happy.
It's trickier with our teenagers. They pick up on things. Unless they ask we don't plan to bring it up. We aren't "out" but someday I very much want to be. once there is some good mileage on our relationship we will talk about what to say. For now we're still dealing with our own feelings.