Is there a cough medicine I can buy my 6 year old that actually works?

Dextrometorphan. If anything works otc....It is the aforementioned active ingredient. Honey has also been proven to sooth the cough receptors, but don't have high expectations! having said that, as pediatricians, we don't like cough supressants, unless the cough interferes with resting at night. As annoying as the cough is, it's there for a reason, it protects the lungs from all the phlegm that drains postnasally.
Not really. The evidence behind most cough medications, other than narcotics, is not very good. We often prescribe a number of cough medicines without expecting much in return. May want to consider home remedies (like decaf tea with honey); they are just as likely to work as the over the counter medications, without the potential side effects.