I'm talking more about the incidentals and of course the souvenirs the kids will want.
We'd need more details about your trip to give you accurate ideas. For example your transportation makes a difference in tipping.
Incidentals:
Tipping for town car driver, bell hop, maid, and restaurant waitperson.
If you'll have a rental car-gas.
If you're driving or have a rental or a car service that allows a grocery stop it's a good idea to pick up snacks, gum and drinks (lots of water) at a grocery (not Goodings).
Snacks in the park: Water $2.00-$2.50, Hot Dog $3.29, Fries $1.89, Coke $1.99 & $2.29, Bagel $2.29, Yogurt $2.49, Ice Cream Cone $2.79, Turkey Leg $5.19, Dole Whip $2.79, and Mickey Bar $2.39 in restaurants. For more ideas hit the menus on here.
Budget in a little (ok quite a bit) for things you might forget at home like extra film, batteries, Tylenol, mole skin, sunscreen or rain ponchos. These items can be costly to pick up in the parks.
If you like to take photos don't forget the cost of developing your film.
Ride Photos $15-$20
Flying? Long term parking for your home airport
Souvenirs:
IMHO I'd let the kids earn their own spending money (but you're trip is coming up pretty quick) but in your case give them a set dollar amount to spend however they want with clear instructions that when it's gone it's gone! DH and I buy 1 snack a day and buy them a shirt/hat the rest is up to them. How much you give them is up to you. Our last trip each DS was given $100 in addition to any money they may have saved from Bdays, Christmas and report cards.
If they collect something specific maybe post that on here and someone can give you an idea of the cost at WDW. For example pins, hats, cups, snow globes, trains, Legos, Christmas ornaments......the list is endless. Cheapest collection to start is the smashed pennies. Cost is the penny itself and 2 quarters to use the machines. There's a website that you can print a list of machines in WDW (actually all over the USA too) and WDW sells a penny book now too. There are also a few quarter machines too.
In Epcot the kids passport can be a great learning tool. Cost is around $10 I think. At each country the kids can meet and talk to a native of that country, encourage the kids to ask questions, then the CM will write a message in the passport in their country's language and a translation. Some people suggest the kids ask each CM to write a specific phrase that might apply to something at home or school.
And don't forget yourself and your DH. Do you collect anything? Last trip DH bought a license plate at MK and decided on the last day he wanted them from each park! We did 3 parks in one day to get him the other 3.

Plus he bought a Test Track and WDW too. They now all hang together in the garage.
What about gifts? Do you have anyone that is maybe watching your house or pet that you might want to pick up a gift for them? Or someone that has a Bday coming up? How about a little Christmas shopping in July?

Disney ornaments are really cute and some really expensive.
Holy cow this is long.

ETA At your kids ages you might just sit down with them now and be totally frank with them about how much "junk" errrr stuff there is to buy in WDW and set the budget now. Ask them if they already have ideas and need price ideas from us.