Crocs are very popular in my area, with kids more so than adults. At my dd's school, half the kids are usually wearing Crocs (or croc-offs) at any given time. Many have multiple pairs and they like to switch the heel straps around, and wear one of one color, one of another. My 8yo wears them nearly every day, unless she needs tennies for P.E. or if it's snowing, and the 3yo wears them almost as often. They have both walked all over WDW for a week straight, DL, and local amusement parks, zoos, etc, wearing them in complete comfort.
Here, they are available in mall kiosks, Hallmark stores and department stores, but they are often low on the popular colors. To make sure you get the color and size you need, I recommend trying them on in a store to figure out the right size, then ordering them on-line if they don't have the color you want in your size.
Good luck and have a great trip!