My 4 year old has a terrible hacking cough-what's the best treament?

I have tried Robitussin DM, Sudafed, Benadryl, Cool mist humidifier, ludens, honey and lemon. I am just at the end of my rope! It's not junky, just hacking-like she has a tickle in her throat. It's not really bothering her now, but she can't sleep and it's gagging her. :( Any suggestions? She's about 35 lbs.