My Husband And I Unknowingly Both Gave My Son A

My husband and i unknowingly both gave my son a pill today. He is 5 1/2 and weighs 45 lbs. He takes 1 20 mg pill. I was wondeting true dose of kid his size. I think he needs more than 20.... but he got 40. What would have been effect to him?? If got 2 pills?j