Magyk by Angie Sage

Silas went in with a heavy heart. He saw Sarah surrounded by six white-faced little boys, all too scared to cry.
"She's taken him," said Sarah hopelessly. "Septimus is dead, and she's taken him away."
At that moment a warm wetness spread out from the bundle that Silas still had hidden under his cloak. Silas had no words for what he wanted to say, so he just took the bundle out from under his cloak and placed her in Sarah's arms.
Sarah Heap burst into tears. (6)

The night that Silas and Sarah Heap's seventh son Septimus was born, Silas found an abandoned baby girl in the snow. When the midwife wisked away the dead body of Septimus, Silas and Sarah decided to keep the girl, and named her Jenna. Little did anyone know that Jenna would play a role in a battle between good and evil ten years later. Because Jenna might be the key to overthrowing the man who killed the Queen and the head wizard ten years before, on the same day Jenna was found. With the help of some of the Heaps, the current head wizard, and a mysterious Boy 412 who deserted from the enemy army, Angie Sage begins the series with Magyk.

AMY