I have a 15 yr old girl and she got her first cell around 10 yrs old or so. We have ATT and added her to my plan.
Our primary reason for adding a phone was safety. She had it at sleepovers, etc and could text or call us anytime.
Her first was a little motorola flip which worked out fine. At middle school age, she got a motorola razor flip that was a good phone, however this was the age where 1 got lost, 1 got dropped in the toilet and the final one stayed around till 8th grade. We had the insurance plan with ATT and when one was damaged, lost, etc....we only paid $50 to get a replacement.
However, in 8th grade, she got her first IPhone. It was great and worked until beginning of 9th grade and she had her eye off of it for a second and it was gone. IPhones are NOT on the warranty plan with ATT but you can buy a plan for about $70. About 6 weeks after replacing the IPhone, she dropped the new one and while she was talking to me and crying, she dropped it in the toilet. The final IPhone was covered by the warranty we purchased and instead of paying $400+ for a new one, we were able to replace it for $199.
I would suggest starting small with your investment because they will lose it or break it at least once. My daughter does more texting than talking so that is an option as well. We have unlimited texting plan because we both text a lot.
One plus she has with her IPhone is that instead of buying the calculator the teachers wanted her to have for Honors Algebra at $140, she bought an app for her phone $4.99 and it is also used for that purpose.
I realize your child is not at the age for an IPhone yet, I think the way you are approaching it as in starting small...is a good idea. Also, with texting....just like facebook..have the codes for the phone so you can check texts on occasion.
Good luck!! They grow up TOO fast.