Accidentally gave my four-year-old a double dose of childrens mucinex cough and cold. no symptoms so far, was given 15 minutes ago.

Should be ok: This is one reason the government advises against using cough and cold medicines in young children. The benefits are small, and sometimes kids get too much medicine. That said, a one-time double dose of such medications should be ok if the child does not take another dose for the next 12-24 hours. Temporarily, antihistamines may cause drowsiness, and decongestants may have a caffeine-like effect.