Is it healthy for an 11 year old to go on the master cleanse?

NO WAY! An eleven year olds body is still working on the process of maturing into an adult. If they were to do something as extreme as "cleansing" it would seriously effect their growth and would make them really really sick, it isn't something to consider. You should always talk to the doctor to learn more about healthy and safe ways to lose weight. If weight even needs to be lost.