I know it may not be what you want to hear, but if the weather on the west coast of Florida is anything like the weather in South Florida, you're taking a real gamble in planning outdoor activities in the afternoon. Just looking at the past month, rainy afternoons have been pretty much a daily event in Miami. Our weather has been very unpredictable as I have found myself drenched after an Everglades hike that started 30 minutes prior with bright sun and cloudless blue skies. With rain usually comes lightning, and mosquitoes, which usually start feeding after 5 p.m. With that in mind, assuming the weather in Land O Lakes is similar to ours, a morning birthday party is the safest bet in the summer, not just to avoid the intense, burning sun, which is strongest from 12-2, but to avoid rain and the mosquitoes later in the day. If you must, plan a party 11-2, it is usually around 2-3 that it starts to get menacing and rain (at least in Miami).
I also second Ena's suggestion for a bouncy water slide, again assuming it gets as hot over there as it does here (average of 92-95 or more in the daytime). You could save some money and use it for the clubhouse rental by avoiding the bouncy house and purchasing a sprinkler hose attachment so the kids can get wet and an inflatable pool from WalMart or this http://www.walmart.com/ip/Inflatable-Tunnel-Water-Slide/2..., or even a slip and slide, like this, which can be fun if kids start racing: http://forestry.sfasu.edu/gallery/v/pwcc/Activities/Slip+.... I am sure all of these options would be a lot cheaper than the cost to rent a bounce house, which I am told is usually about $200+. Good luck and have fun!