How often is blood test needed for child on tegretol?

Probably often early. In adults, because of adverse affects, like temporary loss of blood platelets, blood testing is done frequently during the first few months, and them 3-4 times a year. It is important to know that stable concentrations of this drug are attained 2 to 3 weeks after it is started. Being that this is a child, the dosage and testing may be different but similar. See your doctor for details.