In the British royal family there is a clear rule about smartphones: no mobile phones for the children. But Prince George (12), who celebrates his 13th birthday on July 22nd, is said to insist more and more on his own device. As his father Prince William (44) told during a trip to Brazil, the topic is now becoming a real test of patience: "It slowly comes to a point where it becomes a somewhat tense subject. But I think he understands why – we e…