Over the counter cough medicine, even those with real, active ingredients, do not appear to be more effective than the placebo in a controlled test.
http://adc.bmj.com/cgi/content/abstract/86/3/170
You may as well tell the kid the spoonfull of piss your giving him is actual cough medicine. It will work just about the same as long as he believes it.
spoiler (click to show/hide)
Stolen from this week's Skeptoid--one of their best eps ever--in which the placebo effect (and how it's not always a bad thing), is explained in detail
http://skeptoid.com/episodes/4151