I think this is incredibly ambitious and a very large group for young kids. But you say they are cousins and used to sleeping over, so I'd be sure to have a lot more supervision than J. you and your mother. And I doubt you and your mother will be doing any "sleeping" at all.
Those beach tents are fun but if they have curved poles to hold them erect, remember that those pole points will scratch your hardwood floors. Put blankets down under them, or stick furniture protectors on the bottom and hope they stick.
I wouldn't try to make them more "girly" - that sets the kids up for all kinds of feminine stereotypes like "camping isn't fun" which can lead to the tearful homesick stuff another mom posted about. Instead, put a couple of sticks through the tops and put pictures of birds or owls on them, any kind of woodsy nature-y props you can find. Cut stuff out of magazines or hunt in the yard - don't spend any money. The kids are only around 6 years old. But don't go crazy because the kids won't care. Get a couple of cheap lanterns at the dollar store (be sure they come with batteries or you will go broke with those!) and hang one inside each tent. Have a system in mind to "assign" kids to each tent because otherwise it can get clique-y as they choose some and ban others. Maybe assign them numbers, and each tent has a 1, a 2 and a 3? Something like that. Then move them around.
They can sing campfire songs - you can probably get a CD of them at the library. If you serve marshmallows as someone else suggested, be aware of the dangerous trend among kids to see how many they can stuff in their mouths. There have been some deaths because the marshmallows soften with body heat, stick together, and block the windpipe. So definitely supervise!
If you have a lot of open space (which it sounds like you do with unfurnished rooms) then engage the kids in organized games - use the old fashioned games that kids play at camp. You can get a book with ideas and rules from the library - they all use cheap or standard household props, and of course the book costs you nothing. We did this for our kids' parties for years, and they all loved it because they had never played them!
I think separating the kids by early sleepers/early risers is a good idea. You can do it like a real camp - they have bunks and schedules and assigned chores/roles: the group that will help with breakfast goes to sleep first, the group that does clean-up goes to sleep last (and that will encourage more of them to go to sleep first!).
If you have a pool, you absolutely need a bunch of parents to supervise, and do not count yourself as you will be answering the door and dealing with the kitchen.
Kids can J. sleep on the floor unless you have some air mattresses and some cushions from chaise lounges. Borrow from cousins and neighbors - don't pay for new stuff.
Consider a movie to quiet them all down later at night. J. serve popcorn and some pre-made s'more snacks (sort of like rice krispie treats).