With a party of more than 2 adults, some sort of communication device is definitely helpful if your party decides to split up for a short time. I believe most of the major cell carriers work fine in WDW. If you decide to get the radios, however, then make sure they have the sub-channels, which will help prevent cross-talk with other people not in your party. We had the Cobra FRS which worked great at WDW. We just bought Motorola Talkabouts but we haven't actually used them at WDW yet. I can't tell you personally how well they work at WDW, but I have read many positive reviews from other DISers. The 2-mile range FRS radios work fine, however if you get 5-mile GMRS range radios, remember they do require a FCC license, which is $75/immediate family and lasts for 5 years.
Hope you have a magical trip!