TarraLee, if you'd like, you could take our 3 next year.
Seriously, I think you'll have a blast. Regarding a couple of points you raised:
Stroller - I'm probably beating a dead horse, but I don't think there's any way you would need one or want one. The kids you are taking sound relatively adventuresome, what with the tubing and four-wheelers, so I imagine they are also energetic and will not have a problem keeping up.
Poly - We just got back from the Polynesian, it was our first stay there and we loved it, it is very convenient to the MK and really to EPCOT when compared to some other resorts. Kids love the monorail, and the little boat to the MK is nice too. Not sure whether you would swim in December, being more southern than us yanks, but the slide at the Poly is cool, as you probably know. The kids will definitely do that like 100 times. Also, it's a better place to rent a watermouse at than many other places, since the lake is bigger than most and the views are cool.
Pass-through - Like I said, they sound adventuresome, so it's probably unlikely, but yes, you can do the pass-through for scared kids too, they don't have to be little. My oldest is 8, but my least thrill-seeking, and we almost passed him through on the Test Track. The funny thing is, we talked him into it, and now of course, he says it's his favorite ride and talks about it like he never had any second thoughts about going on it.
Mid-day nap - I dunno, it could work, but I doubt they will actually sleep, they might be too old. I guess it depends on your schedule. Just leaving the park, going swimming at the hotel pool (another plus for the poly, being convenient to the MK and EPCOT) for a few hours, then going back might get you the break you need. I know even my youngest (just turned 4) would not likely go to sleep midday anymore, not at WDW!
Someone mentioned the campfire at Fort Wilderness - we did that this time and the kids did love it. Again, it's relatively convenient from the Poly - take the monorail to the contemporary, then the boat to FW, then walk or take a FW bus to the campfire site. You can buy smores packs for a reasonable price (as Disney prices go) at the "Chuckwagon" there. It's free otherwise and fun for kids.
When we go, we all wear matching shirts - it makes it easier to keep your eye on them. Even all wearing the same color is helpful. It looks good in the pictures too.
It might be harder for you not being the parents to plan something like that though.
Be prepared for them to occasionally be brain dead and do something completely unpredictable, like step out of the monorail just before the doors close - that happened to us this last trip with our youngest. The story someone shared about the emergency brake is something that I can see happening especially when you aren't really able to anticipate their actions as well as their parents might be able to.
Kids get wet sometimes more easily than adults, so you might consider bringing something for them to change into or having them wear a swimsuit as shorts (for boys). All those cool play-in fountains at EPCOT and DD are irresistable for kids, so either anticipate and prevent their entry or plan and let them get wet.
Have fun!
She is tall for her age though, I think. You will have a blast doing this with kids. Everything in life is different when seen through a child's eyes.
