We own at Oakwater Resort - right by Windsor Hills - I would plan to be at the airport at 8:45Am - with two little kids, I would make that 8:15 - 8:30.
I would probably leave at 7:30Am
It takes about 30-35 min. to the airpor
I think this is correct. I think the person who does it in 20 minutes might not be factoring door to door.
Length of time to return the rental car is dependent on the rental car company. If it's one of the on site companies, it's pretty easy and usually pretty quick, but I would still allow 15 minutes; I can't speak to off site - but I would imagine you need more time.
How many bags will you have?
Sometimes, my DH would drop off me and DD with the luggage, we would use the skycap and he would drive around and drop off the rental car and meet us at the gate. Of course, that was only with one child, not two. But the walk from the car rental back to the terminal is a bit of a ways; how did it work on your arrival? that should help you gauge what to do on departure (as far as juggling bags and children)
What day of the week is your 10:10 flight? Monday mornings can be brutal because of all the business travelers. So security can take longer than usual.
Good luck! and don't forget change for the tolls!