Is there anything over-the-counter that I can give my 18 month old for a cough?

Not really. The short answer is "not really" since all the cold remedies are listed for 4 years and up. That said, you can give honey, which is a natural cough suppressant. That's why tea with honey is so effective. Keeping them well hydrated works too, and vicks at night either with a rub or a vapor bath helps. Sometimes just getting the bathroom steamy and sitting in there awhile helps.