Mellsey, are you MY poly couple?
That's completely my situation. Right down to how far apart we live. For the last year, every weekend we pack up and move into the other couple's home, going home only when we have to. I tried to discuss polyamory with my 14yo son once, and he wouldn't talk to me for hours, so I can't help with telling the kids about it. Right now wife#2 and I get together once or twice during the week, but we're homeschooling and that will change in the fall so I know things will get harder. We call the other couple almost every day and I facebook with them all the time. I doubt I have any good advice for you, but do know that I sympathize.
Here is what I hope: Gradually, over time, we will become more affectionate in front of the kids. I hope that if we don't show any shame about what we're doing, our kids will grow up knowing that this is a natural, loving relationship, and then we can tell them.