The cheapest way for you to stay on-site is at Port Orleans Riverside, it's the only Moderate Resort that sleeps 5 people - it's about $100.00 less per night than the cabins. The next best choice is getting two connecting rooms at a value resort. We are a family of 6 and that's what we always do. That way we have plenty of room and most importantly, a second bathroom. All-Star Movies is my kid's all-time favorite resort. Super fun!
GET THE DINING PLAN! There's no way I could fee my kids 1 Table Service/Sit-Down/Character Meal, 1 Counter Service/Fast Food Meal and 1 Snack per day for about $11.00 without the plan.
Also, since 2005, Disney has run an annual promotion in September for Free Dining. I would say this year's fall discount should come out in the next few months. It's a huge discount, especially for a family your size. You can book now, at any of the Disney resorts, for $200.00 down/deposit and you don't have to pay the balance, or even another payment, until 45 days before your arrival date. If you go ahead and book your resort, when the free dining discount code is released, you can call and have it added to your existing reservation.
The week of Labor Day is probably the lowest of the entire month of September - very low crowds. The crowds towards the end of the month are noticably higher with people coming in for the opening of the Food & Wine Festival. We have been in September twice and the one thing you really have to prepare yourself for is the VERY HOT weather. The low crowds and discounts are worth it for us to travel in September, but it's super hot.
If you haven't been to Disney before, especially with kids, I highly suggest that you purchase The Unoffical Guide to Walt Disney World and join their companion website,
www.touringplans.com I use their awesome crowd calendar to plan all of their park days. Also, if you are going in early September, your Advance Dining Reservations window has probably already opened up. Even if you aren't taking advantage of the free dining promotion, thousands and thousands of other people will be and the restaurants will be packed and will most likey not be taking any walk-ups. The website
www.allearsnet.com has a great comprehensive list of all the WDW restaurants and menus.
Here's a list of tips I usually give to people new to WDW:
1. Make sure you have Advance Dining Reservations.
2. Get to the parks at opening. You really can get a lot done in the morning because so many other people sleep in.
3. Eat lunch around 11:00 or 11:30 a.m. before the fast food places get busy. Then when while everyone else is eating lunch, you will have lower crowds.
4. I buy a ton of the two for a dollar ponchos at the dollar store and keep them in the backpack. That way you can throw them a way after use and not worry about keeping up with a wet bulky poncho the rest of the day. Bring throw away ponchos to wear on Kali River Rapids at Animal Kingdom, if not you WILL get soaked.
5. Other things I put in the backpack each morning are Tylenol/Motrin, small first aid kit, camera with extra batteries, small hand towel, hand sanitizer, wipes and a bottle of water.
6. Most single service items under $4.00 are considered a snack credit on the dining plan.
7. At Magic Kingdom always head to Fantasyland first, if you are not doing Fantasyland on a particular day then head to Splash Mountain first, then Big Thunder Mountain; at Epcot always ride Soarin' first and get a Fastpass for later after you get off of the ride; at Disney Hollywood Studios always ride Toy Story Mania first, then head over to Rockin' Roller Coaster and Tower of Terror; and at Animal Kingdom ride Expedition Everest First then head to the Safari.
8. Favorite fast food restaurants at MK are Pecos Bills and Cosmic Rays; favorite fast food at Animal Kingdom is Flame Tree Grill and Pizzafari. If you are there while it very hot, go to Pizzafari.
9. If there are two lines going into an attraction, always take the line that goes to the left.
10. My best tip would have to be USE THE FASTPASS SYSTEM. Our rule of thumb is if the wait is 25 minutes or less, we wait in line. If the wait is 30 minutes or more, use a Fastpass. It is not highly publicized, but your Fastpass does not expire until the end of the day. You can never use it before the return time stamp on the pass, but you can always use it after. We try to collect Fastpasses during the early part of the day and then use them later in the afternoon when the parks become more crowded.
Click on this link created by fellow Diser, Doconeill, and it will give you an excellent overview of Fastpass. It is from Doconeil over on the Disboards and super helpful:
http://www.oinc.net/disney/fastpass.php
If you still don't understand, just let me know your specific questions.
11. I like the Photopass. You can pre-buy it before your trip for $99.00. When you enter the parks you will see the staff photographers, they will take your pictures and give you a Photopass and you can use that for the rest of your trip. Each time you see a photographer have them take your picture and they will scan your picture. Then when you get home you can pull up the pictures, with the code they email you after purchase, and pick the photos you want on a CD. They will then mail you the CD and the copyrights to the CD and you can take them to
Walmart or Walgreens or wherever and get them developed. They also have a lot of cool software that you can use to edit your photos before you have them put on the CD.
My favorite Disney planning sites are:
www.disboards.com
www.allearsnet.com
www.wdwinfo.com
www.intercot.com
We all plan to different degrees around here and September is the height of free dining season and that's one very important fact that can highly affect your trip and planning for it can greatly affect the happiness factor of the trip. 
