FW Cabins are now our favorite place to stay, having replaced our beloved Poly. We stayed there twice last year and obviously loved it. I'll try to answer as many questions as I can remember:
Size of FW Yes, it's very big but with the bus system, it's very user friendly. Make sure to study the bus routes when you get there to learn which you want to take to go to different destinations. The busses all have a color, which indicates which route they go on. They are also designated either "Settlement" or "Outpost". This will tell you which direction they are going - either in the direction of the Settlement, which is where the marina, restaurant, Hoop Dee Do Review, etc. are located, or it is going toward the Outpost, which is where you catch the bus to all parks except the MK.
Children's activities There are LOTS of things to do at FW. As indicated, the main pool just opened back up and now has a slide. They're also working on a splash area which should be open within a month or so. There also is archery, horseback riding, lots of playgrounds, tether ball, the beach, organized games that are played at the pool and in the field near it, the campfire and movie at night.... You could really fill a vacation having fun at the Fort, but of course you might go to the parks some too....

Oh, and there is also the pool in the cabin area too, but it doesn't have a slide - just the main pool now does.
Golf carts - Very fun and definitely worth it if you can fit it into your budget. With a GC, you really don't end up taking the internal busses anywhere. There is GC parking anywhere within the fort that you might want to go - both outposts, the restaurant (Trail's End - a buffet), both stores (the "trading posts"), the pools. Some of the GC parking even have outlets to recharge your cart (I know the parking by the marina does) so you can have a full charge when you get back to the park. You can also charge at your cabin. I'd plan to plug it in at least every night so you don't run low.
Which loop - I've stayed in 2100 and 2700 and for us, we loved 2100. It's the one that is closest to everything except the Outpost bust stop. It's also adjacent to one of the camping loops, so if you needed an extra shower or to do laundry, you can run over to their comfort station. We did this once and it was very handy. (I don't think there's a rule against that, but I'm sure someone will correct me if I'm wrong.

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Check out the map at
http://www.disneycampers.com/Fort_Wilderness/FW MAP.htm . It will show you where all of the loops are in relation to all the rest of everything.
Smoking - someone asked about this. Yes, I think you can smoke on your deck but not inside the cabins. I'd be very careful of disposing of the butts, though, as I think Florida is still very dry.
In general, FW is a GREAT resort and I think you'll have a fun time. If you get a GC, make sure to go "looping" - checking out all the camp site loops. Some people stay at FW for extended periods of time and decorate their sites throughout the year. Halloween and Christmas are particularly impressive, but they have decorations other times too. The cabins generally don't decorate, but I always take my rope lights to put on the deck. It's fun coming home late at night to be able to see the cabin well before we get there. Here's me from our Thanksgiving trip last year:
The deck area is so nice to have, too. We hang out there while the kids are decompressing from a big day inside sometimes. (Wine tastes extra good at the Fort!) Everyone there is very friendly too - you'll definitely get some waves as people walk or cart by.
If I haven't bored you to tears, feel free to check out either of my FW trip reports (in my signature below). They have lots of thoughts on our impressions and love for FW as well as a good bit of pictures too.
Have a great trip! I'm very jealous of anyone who is going to FW sooner than May 2010!
Leslie