And now for a completely different perspective:
Our family's Disney World motto is: "Time is money!"
We have also found, after 6 trips to the World, that paying for some key services/experiences makes a HUGE difference in the smooth sailing and enjoyment of our trip.
We use Tiffany Town Car for service to and from the airport. The value of getting off the plane, going down to baggage claim, and seeing the neatly dressed driver holding up a sign with our family's name on it is beyond measure. ESPECIALLY with your nighttime arrival and with 2 children, this will be a very wonderful thing.
The driver retrieves your luggage, loads it in a big, comfortable town car, and whisks you to your resort. You don't have to find anything other than the baggage claim area, you don't have to wait for anyone or anything, no lines, no other stops along the way (unless you want a grocery stop, and that's your choice). You now have your luggage with you when you check in to your resort.
On the return trip, because one private driver is picking up your family and taking you directly to your airline, you don't have to queue up as early as you would if you were taking DME. Again, the driver handles your luggage (unless your airline participates in the program where you can check your luggage directly at the desk at your resort -- some do that). At the end of the trip, after you've spent however much time at WDW surrounded by other people, waiting in lines, waiting for buses, dealing with multiple stops before arriving at your destination, it is heavenly to be able to get to the airport in a relaxed, comfortable, quiet manner.
Given the overall cost of a WDW trip, our family believes that the benefits of private towncar service (and we LOVE Tiffany) totally outweighs the cost (about $100 r/t).