I know that bama_ed is a wealth of info.
I hear bama_ed is full of it.
Just sayin'....
Seriously, please don't single me out. We have a whole board full of posters who are helpful and more knowledgeable. Personally I hope several people respond to you with their opinions and ideas.
So the way I hear your plans, you want to rent a popup for about two weeks at the end of December for 7 people (possibly 8 when the oldest is there). Since you asked my opinion, here it is: I think that would be a tight fit. Some popups rented by delivery/setup services say they sleep 8 folks at most and they CAN, I just don't think you would WANT TO for 14 days. That usually means dinettes or couches/gauchos have to set up EVERY night with pillows, sheets, and blankets, and then put away EVERY morning. There's always someone who wants to stay up late and can't because everyone wants to sleep and then there is someone who wants to eat breakfast early and can't because the dinette has someone sleeping on it.
Also, popups are notoriously short of storage space for luggage etc. I know you guys are a family and even though the kids probably know how to push each other's buttons and you as parents know how to keep peace in the family, I think it would be uncomfortable for you. I mean if you HAD TO do it then you could but it would not be preferable.
I would suggest you look into renting a travel trailer that has a bunk room in it where you can put 4 kiddos. They come with a big bed usually and a couch or a dinette that converts to a sleeper. It's a little more expensive but you have more space, more storage, and the ability to close a door or two for privacy. (You can send the youngsters to bed and the older ones can watch a college bowl game on tv or something).
If you REALLY want to do a popup one thing you could do to help storage wise is bring a tent to set up next to the popup to store your clothes/luggage (no valuables) and let the kids change in there or at least offload the stuff out of the camper to give you more elbow room. I see you are from Canada but the weather that time of year at the Fort can be really chilly or very warm.
I have a 12 foot popup with a king, a queen, and a slideout dinette and camp as a family of five in it so I have some experience with the things I say above. Ours isn't the biggest popup but I can "scale up" our experience and relay lessons learned.
But hey, you've got time to check things out, ask questions, and make a decision. So I REALLY hope other folks will share their thoughts.
Bama ED
PS - I think the Fort is a perfect place to do as you wish which is to give the kids (at least your older ones) a place to have a little freedom around the resort. They can visit the Trading Post, the pool, fish, find the old Fort Wilderness Railroad tracks, or just explore and you parents don't HAVE to be with them.