You're going to need money for tips (waitstaff, bell hops, mousekeeping, etc.). Tips for restaurant servers can be charged to your room, but others you will need cash for. You can follow the links
HERE for discussions of tipping levels.
You don't say what type of transportation you be using during your trip. If you take taxi from the airport, this will be approximately $60 each way. Most taxi rides within WDW are between $12 - 20, should you choose to use these.
Your biggest other expense will likely be souvenirs. If you click
HERE, you can see a little video my daughters and I made about souvenirs at WDW (it's called Magical Souvenirs). This shows you some of the types of things you'll find at WDW. I have to tell you that we filmed several more categories of souvenirs that were trimmed from the video for length, so in addition to what you see here, there are many other types of toys, housewares, pet supplies, and virtually everything else under the sun for sale at WDW. Particularly at Epcot, there are many non-Disney items to buy: china tea sets in England, perfume in France, Helly Hansen outwear in Norway, rugs in Morocco, etc. The cost for items like these can really add up.
But for the basics, here are some prices you might encounter:
Plush toys: about $6.00 and up
Children's T's: about $15.00 and up
Adult sweatshirts: about $40.00 and up
Disney autograph book: about $7.00
Trading pins: about $6.00 and up
Light-up glow toys: about $15.00 and up
Ceramic mugs: about $10.00 and up.
Mickey Watches: about $15 and up and up and up.
I hope that this information helps you piece together a budget that makes the most sense for your unique family.
Have a great trip!