This is a fun thing I've seen large groups, actually, even small families do to help keep the group together and easy to spot: All wear matching T-Shirts. You could get them at Disneyland on Main Street and put them on right away, or buy them for use the next day, or purchase before your trip. This might be the best option if you want to choose a really bright color that will be easiest to spot. I've seen fun ones celebrating someone's birthday that were customized with a message like "I went to Disneyland for Erin's Birthday," or "Jane turned 25 at Disneyland."
Matching T-shirts are also useful if, God forbid, your child gets lost. You can easily point a Cast Member to another member of the family and let her know the child was wearing a shirt just like that one. (Having a recent picture of each child with you is also a good idea.) Usually the child has just gotten captivated by something and has wandered a little too far, and an easy-to-spot T-shirt will be a good way to track him down!