A ziplock bag is a must need for many reasons when visiting Disney World...or any amusement park!
One of the best souvenirs that you can purchase for your children at Disney World is pressed pennies. It used to be that you'd have to bring all of the quarters and pennies with you for the machines, but it is much easier to get these souvenirs now with a few dollar bills or a credit card.
There are so many machines throughout the parks, and you are bound to find each person's favorite character.
I typically do a bit of research before visiting the parks about where the machine are located, especially if I know my kids are going to want a specific one - Cheshire Cat for example. The ziplock bag can be easily put in a bag or pocket. You could label one for each person in your family, and all their pennies go in the respective bag. My kids will get their pressed pennies out from time to time when we are home, and it allows our family to reminisce about our previous trips.
3. Water bottle
I normally have a small backpack when I go to the parks, and on both sides of the backpack are two water bottles. Hydration is a key component to a successful day at the Magic Kingdom, and I always am reminding my family to drink water throughout the day.
Any time I have a chance to fill up the bottles, I do so. In every line that we are waiting in, I encourage a few drinks of water for everyone. Don't forget that there are water cups available in many locations around the Disney parks, just ask when you are ordering to see what's available. Nothing is more refreshing than a cold cup of water during one of those hot Orlando days in the sun.