I would suggest either a monorail resort (the rooms are big enough for a family of 5), or FW cabin. Staying at the Contemporary is great because it's just so easy to get to MK. You may be spending more time at MK with your 4 year old, and then the teens can off on their own to MGM.
We've had 6 (one infant) at CR last Sept and it was perfect for us. We also had 6 at FW cabins and we absolutely LOVED it there.
Here's how you should plan your trip:
1. Narrow down which WDW hotels you want to stay at
2. Sign up for DING from Southwest Airlines
3. Watch the Codes board for when
Free Dining is announced (last year it was April 1).
4. The first day Free Dining is announced, you need to call WDW and book your hotel.
5. Keep on checking for great DING rates (the good Ding rates for Sept will be out in a couple of months)
6. Talk to the kids about where they would like to eat (pick great places, it will be free).
7. Visit the DIS boards daily to plan your trip (soon you will be hooked...lol)
HTH