Probably zithromax, that's normally a five day 'sript.
We have had issues (specifically when our twins were two....) about getting antibiotics unless they HAD to have them. Now that they're 5, and with a new one, our ped is a little more willing. Haha, he knows my kids are sickies, apparently!
OP, there are doctors that you can see there if you can't get help right away before you go. Check with your health insurance plan to see what would be covered. I know, I just added to your to do list, when you have a sick kid and enough to do to get ready!
Good Luck! (as a by the way, avoid omnicef if they give you an antibiotic. You don't want to deal with those intestinal issues at WDW. BTDT, a year ago now with 2 five year olds. we ran for potties, and sometimes didn't make it....and they were totally potty trained. It was awful!)