A lot of your budget depends on your family's expectations. Do your kids have at least some of their own money for souveniers? Do you and DH have an evening planned away from the kids? Have you already budgeted for your expenses on the road? Are there character meals planned? Since you'll have transportation, I would strongly suggest snacks and drinks available in your room. There is at least 1 grocery store we frequent that's close to the Dis property, Publix on s. International drive near rt. 192 (Irlo Bronson Hwy). Trips back and forth to vending machines and food courts can eat a hole straight through your wallet.
I don't think it would hurt to have a discussion first, with your husband about the spending money, then the 2 of you with the children to talk about budgets, shopping, and how they will/will not purchase "non-essentials".
Ha Ha I wish I could say how much you'll need. Over the years it became more economical, as my daughter babysat/did odd jobs for her own spending money. Sometimes, though, we found ourselves spending money we shouldn't because we got caught up in the magic.