I am a married mom with 3 kids- 10, 9 & 4. My oldest 2 kids got Kajeet phones when they turned 9. We got them reasonably priced phones & awesomely priced service to go with their phones, just $5.24 a month with tax!
There are NO contracts, and the phones are reasonable (they start around $20). There are a variety of plans to fit your needs.
We have a cheaper plan & just fill their wallets with the amount of minutes we think they will need each month. If they need a little more it takes a couple of minutes to reload them.
You can also pay the extra few dollars a month & get the GPS so you know where they are.