I am going to say that unless money is no object, I would not recommend getting the Dining Plan. My reasons are that this will be your first visit and will be a short one
I respectfully disagree with this simply based on the ages of your children. With kids that age (I have ones that age also) it seems crazy not to get the DDP. This way, you can plan for fun character meals that you would otherwise probably avoid so as to keep a better budget. For adults, I think mostly it is a break even thing (If you don't order alcoholic beverages or appetizers also).
I would also try to visit other parks, not just MK. My kids LOVED DHS -- Star Tours, Muppet show, Indiana Jones, and the Car Stunt show. They also really like Sci-Fi, and the younger two also had a great time at Play and Dine (H&V)--my oldest, not so much! We didn't even do the "big ones" RnR and ToT because of height restrictions, and my oldest just doesn't like "scary rides"
Is there any way that you could expand your trip by a couple of days? The tickets get really cheap after about 4 or 5 days, and maybe even consider a moderate resort? We looked into WL for our trip in January, and it was going to be about $1100 more for the week (for both of the rooms we needed), which is about what our airfare was, so I couldn't justify it since I really wanted to fly to have more time in the parks (also really wanted to avoid 2 days in the car with 3 kids and my MIL!!) My kids loved POR...we did the surrey bikes, and they loved the boat to DTD.
Also, I know this has been answered, but unless you are renting a car, I wouldn't bring a carseat. I really went back and forth over this, since my DD was almost 4, but I knew that on ME we couldn't use it, nor on the regular disney busses. All of the kids did great, didn't try to get out of their seats on the plane. One morning we did take a taxi (van) to beach club for the boys to do the pirate cruise, and we just buckled DD into the middle seat and hoped for the best. I would normally never do anything like that, but otherwise, what do you do, carry the carseat around epcot all day? I don't think it would fit in a locker.
By the time you leave you won't feel so overwhelmed! I bought the Birnbaum book for Disney with Kids, and found the format very easy to read. I also joined TGM for touring plans, and made a folder for each park with whatever info I thought I would need.
I would make your reservations when you feel comfortable with the payments, but prob. by September (even though we did ours for end of January the 2nd week of November!!). I would also watch out for codes and specials...when we booked there was one for a free parkhopper and also added the Water parks and more, which we never used, but we did love the PH, for which I don't think I would have paid $50 each for. I'm pretty sure that you can add these specials to an existing room reservation too,
Hope this helps!!