Redpepper, my situation is a bit different and I don't have a lot of experience,but I thought I'd share what my quad is doing.
Wife #2 and I talked, and we kind of feel like trying to balance the affection obsessively is leaving everyone feeling left out. So we figured that we should all just give affection as much as possible to everyone involved, so much affection that we'll all be sick of it. I don't know if it's working yet, but we kind of think that it can't hurt.
As for kids, between us we have a 14yo, a 10yo, a 7yo, and a 3yo. The 14yo we told about our new extended family, and tried to reassure him that we will all love him. The younger ones we haven't come out and told anything, but I think they have the advantage of not having spent so much time absorbing society's ideas of what a proper relationship should be. We are a little careful what kind of affection we show around them--husband #2 won't kiss me when the kids are around, but we share plenty of hugs. When my 10yo exclaimed that she was going to tell Daddy that I was hugging husband #2 when he wasn't around, husband #2 just laughed and hugged her. We try to keep our reactions light and not act like we are doing anything shameful. The 7yo and the 3yo have made no comments whatsoever about the way we express affection yet.