5 year old positive for strep throat. treated with amoxicillin for 10 days. 1 week later strep returned. now given penicillan. will it work?

Hard to say: Treatment failures do occur, but he could have been successfully treated but re-exposed & re-infected. Penicillin drugs kill strep, so that is not the only question. About 5-20% of the population can carry strep in their throat and never get sick, depending on the season. There may be a family member or close friend as a carrier.