First, that's WAY more than I'd ever spend -- if tickets and food were already covered. Truthfully, if tickets and food were already paid, I'd probably spend zero in the parks -- but I'd want $20 in my pocket just in case. What do you expect to pay for? Let's make a list of possibilities:
- Parking and/or Uber rides
- Drinks and/or snacks ... do you know that you can get free ice water from any food vendor any time?
- Souvenirs ... personally, I don't see the point.
- Are tips included on the dining plan?
- Will you make a grocery store stop on the way to your hotel?
- What else do you think you personally would buy?
Count me in with those who wouldn't be comfortable carrying that much cash around. Since you're worried about over-spending, you might consider buying yourself a $1000 VISA gift card from your bank or credit union ... and using it for all your Disney purchases. You can't spend more than the gift card's dollar figure, yet I think -- be SURE of this -- that, if lost, you have some recourse. Photograph the back of the card so you'd have the phone number to call. I know, I know, the chances of losing it are slim, but it's still safer than cash. If you don't spend it all, you can use it for groceries and gas later.
Actually, if you're traveling with a spouse, I'd suggest two separate $500 cards. It means you could still spend if you split up during your vacation, and it lessens the chance that all your money could be lost.
I personally would not buy a Disney gift card. Why? Because you say it's your first trip, and you don't know how much you're going to spend ... I wouldn't want that much money tied up in a single-merchant gift card. A VISA can be used anywhere.