Hi there, Jill!
Welcome to the Disney Parks Moms Panel. Buying Disney Gift Cards is what I used to do for my children as well, and it always worked out great! Disney Gift Cards are excepted at all locations within the theme parks-- including food & beverage stands and the smaller merchandise carts.
I do have another suggestion for you to consider, one I tried out on our recent vacation during spring break. If you are staying at a Walt Disney World resort hotel you and everyone in your party will be receiving
Magic Bands. As part of the Magic Band experience you can authorize your princesses to make purchases with their Bands. It will always be around their wrists, making it much easier to keep track of. My two oldest children (11,9) loved making purchases this way, and I loved not having to dig for their gift cards.
To utilize this option, just request their Magic Bands be authorized for purchases when you check-in to your resort. You will select a four digit pin code that everyone will use when purchases are made with their bands. It really is very easy, and a lot of fun too!
I hope this information is helpful, Jill! Please do not hesitate to stop by the Disney Parks Moms Panel should you have any additional questions!
Have a magical day!
Faith D.