HELP! Daughter can't stop coughing?!

My 3 yr. old daughter can't seem to stop coughing, especially when she sleeps. She's been coughing off and on since July/August of 2010. She's gone through 3 doctors, who gave her antibiotics and other prescribed medicines that worked for the first 3-4 days and stopped working afterwards. She has gotten her blood drawn, seen an allergist, and had a skin allergy test. We found out that she's allergic to grass but the allergist said there's nothing they can give her at this young age besides an inhaler. So, I decided to get her Children's Claritin, which worked for about a week, and now she's coughing again. One of her doctors told me to give her Children's Benadryl but they didn't have it at our CVS pharmacy, I'm guessing it's been recalled. I've tried cold & cough meds, soar throat lollipops, regular lollipops, honey, tea, water with sugar, and plain water. The plain water sometimes helps relieve her throat for about a minute so she can fall asleep, but an hour later she keeps coughing. This has really gotten bad. She is being sent home from her daycare every other day because she can't take her nap because she's coughing and ends up crying.

What options do I have? What can I do to help stop her cough and have her get a good night's sleep? It's affecting our whole family. She loves preschool and gets upset when they won't let her go back, and I'm a full time college student, and can't go to class when they send her home. I really want her to get better. Any help please?