Hi Mac, well Vistana have full kitchens so you can keep the food costs down a hell of a lot by having breakfast (cereal, bagels and toast is what I usually stick to) in your unit. With two quite young ones, there are bound to be a few days where they just "flake out" and are asleep by the time you leave the parks, having a few things like frozen pizzas etc in the freezer is going to be less traumatic than taking flopped out kids into a restaurant to get fed. Either they sleep in an uncomfortable position, or they wake up and are in a tetchy mood. Whichever it is, it doesn't make much of an experience for mum and dad. The type of rooms you have mean you can get back, stick em in their beds and enjoy a beer/glass of wine and a quick snack together without having to worry about them.
1) If Dollar are at the airport, their desk will be one floor down from baggage collection where you will complete the paperwork ( if you have a new "ID style" licence you should take the paper part as well just to be 100% certain. You shouldn't need it but you never know

) The cars are in the parking lot just accross the road. Youll be given the key and the bay number your car is in. (You'll need to show your paperwork as you exit the Dollar part of the parking lot, so don't pack it away in the trunk

). They will supply you with child seats AT A COST, but in my experience the car seats were not very good quality, and with a daily charge 2 weeks for 2 seats ( both will need em) it'll work out costly. If you need a new car seat, I'd advise buying one in Fl as they are pretty cheap, but you'll need to be covered for your first journey as it is a legal requirement the kids have them.
2) I'm certain it's ok to travel on his mum's passport. But you do have potential problems IF for any reason she had to travel outside of the US without him going as well. Unlikely I know, but not impossible. If you have enough time to get him his own now, it does avoid those type of problems for the future.
3) For kids, there is SO MUCH to do, that just "take your time" is the most important, otherwise much of it can pass you by in a blur. Plenty of film in the camera as the first trip ( particularly with the youngest) is going to be full of wonder for him. OK here are a couple of "quieter things" that are easy to miss but for a first trip worth doing. First trip to Magic Kingdom, take the ferry NOT THE MONORAIL from Ticket and transportation. The monorail is quicker, but having the park come slowly into view, particularly seing Cinders castle and Space mountain come into view psyches them up into a frenzy. With the monorail, you go through the contemp hotel and the park is just THERE in front of you. 3d shows it's worth doing twice ( first time watch the kids, 2nd time watch the show LOL) sometimes the effects can be a little intense for them so that's another reason for watching them first LOL. Smaller kids usually love the parades, so it's worth taking the time to "snag" a good spot, in the front row. If you and your wife take turns to go visit a shop,loo drinks or popcorn breaks the time shouldn't drag too much. Similar with fireworks, it's worth taking the time to get good places because when you're that small it's easy to get your view blocked ( which kinda takes the edge off things. For MK I favour the "rose garden" area that is (from the entrance) to the right and before Cinder's Castle. You also get a good view of Tinkerbell's flight if she's taking to the air. At Epcot we recently watched from the bridge between France and England which worked well. There is also a good spot on the front of the lake ( with SpaceshipEarth AKA the big golf ball directly behind you) but this is kept roped off until about 30 minutes before the show. You need to be inthe area before the ropes a good while before they "drop em"

and it's still a bit of a scramble for the best spot. There are lots of things like the Dino dig, that boys particularly seem to enjoy, so dont get too tied up in the "we must do the big rides" mindset.
4)For eating "out and about" Disney can work out expensive BUT I would strongly recommend looking at at least one character meal. My favourites are the breakfasts at Crystal Palace in MK, Cape May Cafe in the Beach club Hotel or Park Fare 1900 in the G.Floridian, but I know the Garden Grill in Epcot and Breakfastasaurus in AK are both popular. This is a relatively painless ( for parents) way to get to see a lot of the top characters without having to queue up ALL DAY to do so. Have cameras, pens and autograph books ready LOL. Good places outside of the parks that I can recommend are Jungle Jims (burgers,ribs Nsteaks) , Olive Garden ( Italian) , Bahama Breeze ( little more expensive, but big portions and nice food) , Tony Roma's (steaks and ribs) TGIFriday ( same as in the UK but prices are the same in $s as they are here in £). All of those are in the Crossroads area ( which I think you'll be near, depending on which Vistana you're in) they all, I think, have branches on I-Drive as well. The 192, is pretty close to you as well and there are HUNDREDS of restaurants along that road, you'll recognise many of the "brand names" I'm sure