marie claire magazine STYLE IN PARADISE July 7, 2010, 12:00 am Abigail Haworth marieclaire It sounded like a dream getaway. "You're going on holiday to the desert to meet other girls and eat sweet food," Tijanniya Mint Tijani's mother told her. Tijani was excited. "She said that by the time I returned home, I'd be a beautiful woman." Ten days later, Tijani, 14, an athletic student from Atar in the West African country of Mauritania, is eating breakfast with five other girls, aged seven to…