We brought DH's walkie-talkie's from work with 500 channels a few years ago and couldn't find a single open channel. Now we just use our cell phones instead because we have sooooooooooo many minutes and the local area's are so large that they include WDW now.
Instead of the walkie talkies might I suggest the Trac phones cell phones from Wal-mart? At $20 including 20 minutes they are dirt cheap. I just got them for my little kids to use if they need me while at camp and I don't think you can beat them for the price. They have every bell & whistle on an expensive phone except a camera. It only takes 3 buttons to get to my name on speed dial -listed as A Home and A Mom to come in first & 2nd.
There is one catch, before you set it up check and see if you live in the same local area as WDW. You will need to define your region when you set the phone up by calling the phone from a landline. If you wait until you get to WDW you can just define that area as your region and then switch it to your home when you get back.
So far we're really pleased and they work everywhere, just like a regular phone except there is no contract so if it breaks or gets lost or broken I'm only out the $20.
Of course, walkie talkie's are fun for kids so I guess it all depends on what your family likes.