Since her parents took her out of the Church at age 12, I think there is diminished responsibility than if an adult were to leave the Church.
According to Wikipedia, her parents joined Wasilla Assembly of God when she was 4. Either way, she is incapable of being excommunicated for joining a non-Catholic church at that age, because:
Can. 1323 The following are not subject to a penalty when they have violated a law or precept:Further provisions exempt from sentence in case of ignorance, inadvertence or error.
1/ a person who has not yet completed the sixteenth year of age;
Not only does Mulshine attack Palin, he slanders the Catholic Church as unjust and uncharitable.