We have ATT Wireless, and in some places I had a great signal, and in others, I got nothing. For example, I forgot to turn my phone off before going on SE, and I managed to receive a call from DH while we were inside the ride(!). However, standing right outside Spaceship Earth (when I tried to call him back rather than talking during an attraction), I got no signal whatsoever. I joked that it was because the ride is sponsored b ATT.
So, it seemed to be really erratic service (but it will depend on your wireless service provider). At AKL, I had no signal, but I had no problem getting a signal at AK.
Hopefully, your child will not be separated from you. However, putting the cell phone number in the fanny pack sure couldn't hurt. At least a person could try calling to let you know that he was OK, and if you have voicemail service on your phone, then you would get the message as soon as you were able to get a signal.
--Kathy