checkout kajeet. It's a service designed specifically for kids. you can program the service to only accept or call certain numbers. I think it has a GPS locator (extra cost). There is no "plan signup" for a year or two. It's a pay as you go service. www.kajeet.com
I'm considering getting one for my 6yo son since if/when he visits his dad - ever - he will be 10 hours away and I want him to be able to contact me whenever he needs to. Not at his father's discretion. Plus, he'll be able to call his dad whenever he wants (not just when his dad decides to call him once every 2 weeks)