Hello to a fellow Rachel!
By staying at a Walt Disney World Resort hotel, having the Disney Dining Plan, and already having your Theme Park Tickets purchased - the majority of your expenses are already taken care of. This is one of the many reasons I love vacationing at Walt Disney World, as I do not have to think about money the entire time I am on vacation. That being said, you will want to bring some cash for other things. It is difficult for me to give you an exact amount, but I should be able to help you figure out a ball park figure that is right for your family.
You will need a bit of cash for tips during your visit if you are dining at any Table-Service restaurants, as gratuity is not included with the Disney Dining Plans. Also, if you receive any assistance from Bell Services you may want to tip them for their services. You may also wish to leave a bit for mousekeeping for their hospitality.
Now on to those souvenirs! There are so many amazing items for purchase in the many Walt Disney World shops including glass slippers, apparel items, jewelry, mouse ears, turkey leg magnets, luggage, home decor, kitchen items, purses, photo frames - I can't even begin to touch on the variety of items there are! I think it best to set a budget of how much you would like to spend on souvenirs and bring that amount with you. We set a spending limit for our boys and purchase a Disney Gift card for each of them at our local Disney Store or Target before our trip. This way they know exactly how much they have to spend, and can keep track of how much they have left as the card balance prints right on their receipts. They also make smarter purchasing decisions when they are in control. My boys are now 8 and 11 and we have been doing this since they were toddlers, and I plan on continuing this into their teens.
Wishing you and your family a magical vacation, Rachel!
~Rachel