I would absolutely purchase two of these immediately - one for each of my kids. I'd load ten bucks on each, set an annual reminder to top up the account, and a bi-weekly reminder to get them to charge it.
Then I'd have them throw it in the bottom of their schoolbags, and remind them that they can use it to call family if they ever have an emergency. If they ever complain that "ughh i hate that brick you bought dad", I can remind them that if they want something extra, they can pay for it themselves.
They have smartphones as well, but they're frequently out of battery, or out of minutes/credit, or just plain stolen. An old Nokia brick would be perfect.