Yeah, I hear you - at that age I kept my kids in their seats on the plane, too!
The earliest that I've ever taken out of MCO is 11:30 - we've been fortunate to make it into port on time every cruise and didn't have problems getting to the airport, regardless of taking rental car, bus, towncar, etc. But, I will say that one time we took SWA home and the line was literally about 2 hours long to check-in. I know that elapsed time because we couldn't check-in right away because we weren't flying out until later in the day, so we sat in the terminal and timed some of the people winding through the line (yes, we were hard up for a time-passing activity!)
With 2 rooms, you'll be able to store the seats with no problem.