We will (are) be getting the outer bands from Arlene, so John Winter is correct. A lot of the mets are saying that they may have to "up" those rain chances before all is said and done. The thunderstorms are already beginning to pop and they are from Arlene, believe it or not. Sheesh.

As for your daughter's birthday party, I would watch the weather over the next day and see what they are telling you. If you have an 80-90% chance of rain, you may want to hold it indoors or you may want to put it off. But, as you know, living in Florida means we can get rain at any time. I'm in Polk county, too.
Don't know if that helped or not (probably not) but my best advice is to go with whatever your gut tells you. No matter what happens, things usually turn out fine.