Another thing to check is your service provider. If it says "Sprint" please put iMessage in the "OFF" position.
You don't need to do this. Just set each phone with a separate iCloud acct, and install and turn on Apple's FindMyFriends on each phone. Then you can check on all your kids from one screen, the FindMyPhone screen.
Why?
The reason they don't fall back to text messages is because that setting is off by default in iOS 7.
Settings > Messages > ~Send text if iMessage fails
[url=http://cdn.macrumors.com/im/macrumorsthreadlogodarkd.png]Image[/url]
Many users are reporting that these failed iMessages are also not falling back to traditional text messages as should occur when iMessage is unavailable. Instead, the iMessages simply hang during the sending process and eventually show as undelivered.
Article Link: Resetting Network Settings May Fix iMessages Not Working in iOS 7
That's not the only thing. A few things were a little bumpy in my iOS 7 transition. Mail didn't have any of my mailboxes until I force-closed it then opened it again.
Other than that, it's ready. The only serious problems are the sometimes ugly icons and UI and the lack of a jailbreak. Come on, I thought they were releasing the iOS 7 jailbreak soon![]()
FWIW, check Settings/Messages/Send as SMS.
I had to turn it on after upgrading to iOS 7.
Hmm, I'm curious though. Does it have to say "not delivered" when it hasn't been delivered? Just sent a text and my friend didn't receive it, even though I got no error message. Do you guys always get the error message when the text hasn't been delivered?