My husband and I accidentally gave our 4 yr old son, who weighs 42lbs, a double dose of childrens benadryl (diphenhydramine) for his allergies (chewable tablets 12.5mg)?

??no numbers?? The only way for me to comment on the dose is if you list what you gave. Two 12.5 mg tabs in a 42 lb kid might make him groggy but shouldn't hurt in the long run.Four tabs would put him to sleep and he'd be out for a few hours. Luckily the med clears the body in 4-8 hours.