My daughter was diagnosed with pinworms. how soon after taking the medication can she expect relief from itching?

Depends: The worms don't cause the itching, so response will depend on some other things you can do. The worm crawls out the anus in the night & lays eggs in the skin folds.The worm secretes a glue like material with the eggs & this glue sets up an itchy rash that triggers the itch & disburses the eggs. If you wash off the anal skin folds with cold water & apply cortisone cream this can help immediately.