I don't see a reason for a child to have a cell phone unless they are going places without you a lot.
We started with a Virgin Mobile pay as you go phone, we put $20 on it a month for 200 minutes. Not alot but its taught him to be careful with is minutes. I also logged onto his online account and blocked all text messages so he couldn't run those up. As long as you reload it with minutes every 30 days the remaining minutes roll over each month.