I've just gone through the pain and frustration of trying (and failing!!) to install iOS 8 via iTunes.
Please save yourselves the bother and install this major update using the Over-the-Air or OTA update mechanism. You'll need a decent WiFi connection and you can update your devices in parallel rather than serially with iTunes. Make sure the devices are fully charged or plugged into the mains and that all your apps are updated for iOS 8 compatibility too.
From the Home Screen, go to the Settings app, then to “General”, and then to “Software Update”
Choose “Download & Install” when iOS 8 update notification appears
The OTA download is close to 1GB in size and requires nearly 5GB of free storage space on your device to install. Figure on 90 mins elapsed time
Please save yourselves the bother and install this major update using the Over-the-Air or OTA update mechanism. You'll need a decent WiFi connection and you can update your devices in parallel rather than serially with iTunes. Make sure the devices are fully charged or plugged into the mains and that all your apps are updated for iOS 8 compatibility too.
From the Home Screen, go to the Settings app, then to “General”, and then to “Software Update”
Choose “Download & Install” when iOS 8 update notification appears
The OTA download is close to 1GB in size and requires nearly 5GB of free storage space on your device to install. Figure on 90 mins elapsed time