Credit cards are accepted when purchasing ticket media. Debit cards can only be used if they have a credit card option which is usually the case if they have the Visa or Mastercard logo on the front. They are processed like a credit card but the funds are removed from your bank account.
I never carry more than $20 cash. This cash can be handy if you need a taxi, tip money or make a purchase from a vendor that does not accept your
Key to the World card.
Most of the time, we charge all of our purchases (food, souvenirs, etc) to our resort account using the Key to the World room card. Sometimes we use Disney gift cards.
You can always pay off your room account with cash, Mastercard or Visa debit card, credit card and/or gift card. I find it less worrisome to use our Key to the World card which can be canceled should we lose it. I also photocopy the back of any gift cards in case they become lost. I keep all credit cards, gift cards and cash in the room safe until needed.
If staying at a WDW resort, purchase your tickets at your resort so you have them prior to going to your first park.
If you are ever in need of cash,
ATMs are available throughout Walt Disney World.