Final answer:
Griseofulvin is used for several weeks to ensure complete eradication of the fungus causing ringworm of the scalp.
Step-by-step explanation:
The nurse instructs the parents to use Griseofulvin for several weeks because it is an antifungal medication used to treat ringworm of the scalp caused by a fungus. The reason for using the medication for a prolonged period is to ensure complete eradication of the fungus and prevent recurrence of the infection. Since ringworm is caused by multiple species of fungi, using the medication for several weeks helps to ensure that it effectively treats the specific species causing the infection.