I am so worried about fighting on our trip too. We aren't going until late Sept/ early Oct, but I'm already stressing about that. We have 4 adopted kids (9,7,5,4) and we have only all been together 8 months. They fight so much I feel more like a police officer than a mother! My mom swears to me that it is normal and that I fought with my sisters like that, but I can't believe we were this bad!
On our last trip, we had 2 of the kids and they were perfect the whole week we were gone. I'm hoping that it will happen that way again.
I try to teach my kids that their family is the most imnportant thing and they need to treat them with respect, but they have a hard time with that concept. For them, family has never been permanent, so they are still adjusting to the idea that this one is. My sisters are my best friends now and I point that out to my kids all the time. Hopefully someday they will get it.
One thing I do is keep little pieces of candy with me. Then when 2 or 3 of them are being rotten, I reward the one(s) that keep out of it. It usually stops the other kids pretty quick. Then I try to catch them all being good for a while and reward them all. That would work at WDW too.
There have been some really great ideas on here. Keep them coming.