I would suggest starting with a guidebook. I like the Unofficial Guide and the Official Guide. The Unofficial Guide will give you more dollars and cents info along with the nuts and bolts of line times, etc. Birnbaums' Official Guide is a happy, sweet read with lush descriptions and Disney trivia. It has much helpful info, and I think the 2 books complement each other. If you don't want to buy them, look at your local library.
You could use those passes by purchasing hoppers for the littlest 2 that are one day longer than hoppers you purchase for yourselves. For example, everyone with old passes purchases 4 day hoppers for a 5 day visit, and for the 2 without passes you purchase 5 day hoppers. Your biggest expense is going to be passes.
For food prices, look at the menus at
www.wdwig.com and
www.wdwinfo.com. The Unofficial Guide has good money-saving strategies for food. This is an area of your budget over which you have a lot of control and is your biggest variable. You could eat at WDW and spend as much on food as on your passports, or you could utilize the kitchen at your timeshare and pack snacks and cut that budget at least in half. It totally depends on how you want to vacation.
Transportation: Are you flying or driving? If you fly you'll need to factor in a rental car and gas. If you're driving factor in gas and hotel stays. Remember to add the food during travel or non-Disney days in your food budget.
Souvenirs: We give our kids the opportunity to earn and save their own money throughout the year. They are responsible for buying their souvenirs and extra Mickey ice cream bars. We buy the some ice cream, but not every day and if they want more it comes from their budget. This helps teach them fiscal responsibility and appreciation. We make sure, though, that they have ample opportunity to earn a sufficient amount. We wouldn't want them to have less than $50 and encourage them to save more. Don't forget to put some souvenir money in for yourself, and add some for the surprises you get for your kids.
Pack to avoid wasting time and money running after aspirin, bandages, moleskin, sunscreen, etc., or you'll get nickled and dimed to death.
Extras to factor:
stroller rental unless you take your own
pictures
film
rain ponchos
Mickey ice cream bars
tips
taxes
pool floaties
sunglasses (if you forget yours)
This is a list that can go on forever depending on your personal preferences, so I'll stop here. You get the idea, I'm sure.
So, take each category, determine the cost, list the totals and add them together. Good Luck!