I am not sure how old your kids are, but my DH and I have struggled juggling all of our luggage, car seats and carry ons to get to the car rental desks. We have four kids now, but it was difficult even when there were only three kids. I highly suggest that carry on luggage is in the form of a backpack so that you have your hands free to cart kids and luggage around the airport. On our last trip I insisted that we pay someone to help us with our luggage. It is hard trying to maneuver everything. I am not trying to scare you, just prepare you. Even though I have used off site car rental companies before, I suggest you use one of the on site ones. If you go to to the official website for Orlando International Airport, you will find a link for terminal maps and you can see where each airlines baggage claim is in relation to the car rental companies.