What? How is choosing to commit a crime against children a mental health issue? (I am asking this honestly, I can not Imagine how committing a crime, can be anything but a free choice by the criminal) and free choices against laws are a crime, not a mental illness.
Even if the crime is committed under drugs or alcohol, they just remove inhibitions, that means the potential for the crime was there all along, and thus alcohol or drug can not ever be a valid reason for lesser punishment.