I really hate that people forget that children are children when they commit heinous crimes. They are treated differently by the justice system for good reason, because they are children and immature and are liable to make ill-judged, poor decisions.
Although the girl did a truly horrific thing which deserves punishment, she didn't actually kill anyone, hence it makes complete sense that she would receive a lesser sentence. It has nothing to do with her being female or because it "wouldn't be fair" for her to serve time in an adult prison. What may have been a factor is not exposing her to adult prison due to the effect that may have on likelihood of reoffending, and I am glad judges consider issues such as this as sentencing someone is not simply about punishment; I want to live in a safe society so I want to know that attempts are being made to lower recidivism rates.
And FGS please stop saying they are "evil", that's an absolutely ludicrous thing to say. Slapping the "evil" label on people that commit horrific acts prevents us from actually looking at how and why this happened and thinking about what can be done to prevent it happening again.