Truthfully, I was hesitant to even allow these functions on their phone, but given the few family members that we are going to make available to them, I think we can monitor it pretty efficiently.
There was a pretty big hurdle in getting it set up though, as it was my desire to have everyone's iDevices (including our iPhones, iPad, MacBook and iMac) on the same Apple ID. Maybe that makes me a control freak, but I wanted to be responsible for all purchases and uses of the ID.
There's surprisingly little info on the internet showing how to accomplish this, so I'm blogging the steps I took to get it to work:
1. Create an email address for your child. Go ahead and store the password info in a new contact on your iDevice. You'll forget it. Don't ask me how I know...
2. Go into your child's device in Settings>General>Restrictions to be sure that FaceTime is allowed. (You should definitely spend some time working through the other Restrictions to ensure kid-safe content.)
3. Still under Settings, go to Messages>Send & Receive (it will display all email accounts associated with your Apple ID). At this point, you'll need to add your child's email address (Add Another Email...). It will ask for your Apple ID Password. Then your child's email address will be emailed by Apple to verify that the email address is legitimate for your Apple ID.
4. At this point, your child's email address will come up as an option you can check for iMessage to be reached by. Uncheck all but your child's email addresses unless you want all of the iMessages to be duplicated on other devices.
5. In Settings, go into FaceTime and add the new child email there also. You shouldn't have to verify the address again, just list it.
6. At this point, you should be able make FaceTime calls and send iMessages (to other iDevice users). You can manage Contacts easily through FaceTime.
I feel compelled to say something about safe use of these devices by kids: we take our kid's devices from them each evening and control when they are allowed to have them the next day. Of course, we snoop around on their devices to make sure everything looks respectable, too.
Good parents snoop, right?