What is the average dosage of adderall (dextroamphetamine and racemic amphetamine) given to an eight year old child?

No "average" dose: The dose is the dose that works with the least amount of side effects. I have some 8 year olds on 5mg a day, and some as high as 30mg a day. This is a medicine where it is very important to monitor the response and adjust the dose, individualizing the dose for each patient based on response, metabolism and side effects. No two people are the same, so thee is no average dose.