There were 11.5 million children under 15 in 2006, and 10.75 million in 2011. That's about 6.5% less. And then the study is not about children who live in poverty as the headline suggests. It is about children receiving Hartz4, as opposed to children in poverty who receive Sozialgeld (tied to the inability to work), or who have a low income as low as Hartz4. (More here, but in German)