My DD got her phone when she was 6 for Christmas. It is on my plan, $10/mo, and many times over the past 3 years has gone unused for weeks. I bought it for her when she was being picked up from school a couple days a week by other mom's who on two occasions had signals crossed and FORGOT to pick her up. The school is very close to our home, and I work from home, so my thought was if she realized they had forgotten her (if she didn't see the other kids) she could call me to run over to pick her up quicker than she could get the attention of the teachers who are loading kids into the cars in car line, get someone to walk her to the office, and then use their phone. In that amount of time I could have been there. This past school year I picked her up each day, so this was no longer an issue.
Also, it is rare that I am out of the house for work during the day, but in the case that I did have to be, I would tell her to turn it on if she did not see me in the car line when she got out of school, as I would call her and let her know that I was running late due to things beyond my control. Again, this has never happened, but it was a comfort to me to know that I could reach out to her if need be.
We live in FL without any other family, and although I do have a neighbor as an emergency local contact, I can't always count on that, so this was just one more layer of comfort that I gave myself!
My entire family in PA (including DDs cousin who have more like a brother sister relationship) all have our same carrier, so she is free to call the whenever she fees like it. She is using the phone a bit more to call a couple of friends. Her phone is a ROKR phone, so it also doubles as an IPOD type device and we have downloaded the High School Musical soundtrack, Disney tunes, and other favorite songs for her to listen to.
It is also good for the times that DD rides her bike to school. She did not do this often this past year, but one day recently she wanted to ride her bike, and was with a group of kids 2 5th graders, one 4th grader (all who ride each day) - and on the way out of our neighborhood she fell - so instead of holding them up, she called me and I went to pick her up and drove her to school. It's nice to again have that extra link of communication.
All this for $10/month.