Ok get this, fiance is a non mehram, you are not supposed to share such pictures or have any such talk as long as you are not married. There shudnt be any doubt in that.
As to the topic, i too agree that kids in their early teens shouldnt be provided with mobiles (same goes for internet). But we cant help it, the kids have to have cell phones and that too the latest ones. If they dont have it they feel left out and are bullied by their class mates.