We just rented a car for the first time at WDW, and it was terrific. I loved not having to wait for the buses, standing in the aisles, carrying the little ones at the end of the night, trying to find room for the stroller and all those packages. Plus, we were able to venture off site for food and groceries which saved $.
Especially with a 16 month old, it would be much nicer to have a car. Getting around the area isn't all that difficult, there's signs everywhere. The parking lot isn't that much farther that the bus stop area either. Keep checking for deals , they change everyday. Mousesavers.com has some good codes for specials.
BTW, with your group I would recommend renting from one of the 5 car rental companies that are onsite at the airport , most of them have online preregistration, Dollar has something called FASTLANE; another one has Emerald Isle club; It makes it much faster to get your car when you've preregistered.
Regarding the parking lots at closing, we never had a problem getting out, but then we were there during a slower season. Try to time it so you're not in the "rush" to leave- stay in the park a little longer or leave a little earlier and you should be fine.