I'm not sure where to begin, but I'll do my best. We're a family of 5 and will be taking our first trip to WDW this coming August. Even though I've never been there, I've learned a lot from so many on these boards. You've already received some great responses. Here are some of the things I did or took into consideration when planning. I'm obsessive about planning and booked our trip in December of 2002! Anyway, here goes:
Airfare: Sign up for Fare Watcher on Travelocity so that you can get a good idea of what a good price would be. I watched fares rise and fall for months before I finally purchased our tickets. I budgeted for $200-250 round trip, not including the associated fees. I ended up paying $273 round trip including the fees which was pretty much right on the mark. I could have gotten a better rate if I would have settled for a connecting flight, but we wanted direct. Look for connecting, the fares are a little lower. Remember to pack a meal for the flight. Many don't offer meals and those that do charge for the most part.
Resort: I would suggest staying onsite. Remember that if you stay off site, you'll need a rental. You won't need that if you're only planning on staying at WDW because you'll get complimentary transportation. A rental runs around $250-$300 for a week, and that's with a discount; you could get a slightly lower rate but not by much. IMHO, you'd be better off spending that on your hotel. You can stay at a value resort or check out one of the Downtown Disney hotels. Also, try not to use a credit card to pay for rooms. I used my Visa check card instead. This way you can basically put the resort on "layaway" and pay a set amount each month so that it's paid off before you even get there. You also don't have to pay interest since it's not on a credit card. Make sure you check with your bank regarding spending limits on your check card though. My bank's cap is $5,000 per day, so I had no worries in that aspect.
One more thing; most of the time it's best to book room only and not a package unless Disney decides to offer another great deal like the original Fairytale package earlier this year.
Meals: Others have already suggested getting groceries for in-room meals. You can get things like cereal, milk, snacks, beverages, water, etc. Don't forget that the food courts at the values have toasters and microwaves that you can use too. You have lots of options here. Include a grocery stop on your way to the resort. Bring a larger collapsible cooler with you to keep perishables cold in the room. Remember that ice is free, and this way you won't need to rent a fridge at $10 a night. Kelloggs has a program where you accumulate "dollars" that you cut out from qualifying cereal boxes. You accumulate them so that you can bid on character meals. I can't remember the name of the program off hand, but just go to the Kelloggs website.
Tickets: If you don't go through a ticket broker for tickets you can get park hoppers at your local
Disney Store and buy them one or a couple at a time so that you don't have the burden of buying them all at once. It won't change the amount you have to spend but it might be easier on you as opposed to doing it all in one fell swoop. If you decide to get UPH passes instead, purchase them ahead time and you'll get a discount.
Spending money/souvenirs: You can buy Disney dollars a little at a time at your local Disney Store. Also, if friends or family are going to be buying a gift for one of your children and they ask you what they want or need, you can politely suggest Disney dollars. Try Ebay if you have something lying around the house that someone else might be able to use or have a yard sale with the proceeds going towards vacation. Set up a holiday account hand allot a certain amount each paycheck that goes into it. Use dollar bills only to pay for things and put the coin change you get back in a jar marked WDW or similar.
Last but not least, check out Mousesavers.com on a regular basis. There are some great ideas on that website and when discounts are offered by Disney, they're sure to be published there.
Have fun planning!