iPhones hold their value... after 2 years most cell companies (or even apple) will buy them back and the updated version will basically be free.
Also, if your kid breaks it.. he has to pay for it.
It's hard to send i kid out on a mission that hasn't learned how to deal with all of the temptations phones bring. (considering for the 2 years they are on a mission they will have to be glued to their phones).