nancy;
If you are asking what time should they pick you up at WDW in the morning to get to USF at opening, I would suggest at least an hour ahead of opening time at USF. The traffic at that hour of the morning on I-4 can be kind of heavy and will slow things down. You also will want time to get to the gate of whatever park you are planning to do first, which entails walking from the drop-off point, through Citywalk, to the park. This isn't too far, but will take at least 10-15 minutes with a large group. I have traveled with large groups before and, no matter how motivated they all are, it takes longer to move a big group than just a family.
You will also need to decide which park you are going to ahead of time if you are planning on buying just a one day pass. Like at WDW, one day passes are good for only one park, either Islands of Adventure (IOA) or Universal Studios Florida (USF), but not both. Check out the Universal website and see what attractions you think your kids will want to do more.
Hope you have a great trip!