What? It's not for girls, it's for "children". I'm not a doctor, but I'm pretty sure that boys are children too.
The book is aimed at girls. It would be nice to think that children were not conditioned by gender stereotypes at the age of 6/7, but unfortunately they are.
I'm going to believe the author when she says that this is for children. I just don't buy the idea that boys can't enjoy stories with female leads. Look at Brave or The Hunger Games. My son loves those.