The first two weeks of August are very busy. The last week is better as the kids go back to school. There is no way you can visit both parks in 1 day with a 5 and 7 year old, they just can't keep up.
I'd say to visit USF since they can't make the height requirements for the big rides at IOA. They would be bummed seeing rides they can't go on yet.
At USF they'll have ET, Barney, Animal Planet Live show, MIB, and the Woody Wood Pecker play area with the kiddie coaster. Next to this is the water play area where the kids squirt water guns and stand under these huge buckets that drop water. Yes, bring a change of cloths and a book to read. Once kids start playing in this area, they want to stay for 3 hours.
And yes, if the park opens at 9am, try to be there at 08:30 so you are first thru the gate. It always takes longer than you think to walk from the parking garage to the park. Visit MIB first, ET, then check the show times.