Look at the attached link which has a map of Fort Wilderness:
http://www.wdwinfo.com/resortmaps/fortwilderness.htm
If you are coming to the Fort from another resort, you arrive either at the Outpost area (top right in the link) or the Settlement area (bottom left in the link). The Outpost area has a bus depot where external busses from the parks, DTD, etc. come in and you can transfer to an internal Fort-only bus. The Settlement area has a bus stop (also external-internal transfers) that comes in from Wilderness Lodge and a boat dock with traffic from MK, Wilderness Lodge, or the Contemporary Resort. If you drive in, you arrive at the Outpost.
However you get to the Fort, go to either the Settlement or Outpost bus stop and catch a YELLOW internal bus and get off at the loop 1400 stop. Note it is a small circle within a big circle of campsites. From the bus stop, take the first left along the big circle and approach the campfire/singalong area from the rear via the sidewalk that goes to the Comfort Station (showers and bathrooms). You can roll up and down the center aisle which is paved and directly in front of the movie screen. The big wooden benches set in several rows on either side have gravel on the ground. So your parents can roll down the center aisle, park and watch there (leaving room for people to walk) or park behind the rear aluminum bleachers and walk to whatever seats you want.
Some paths and approaches to the area do have the wooden bridges which an ECV might be able to pass through but the campfire rings where the wood burns on either side have a concrete walk around them but they are not connected directly to the paved center aisle with more concrete, only gravel. To roll in from the sides like that they would have roll around to the rear anyway.
So if they want to walk a short distance and park the ECV nearby, any approach will do. But if they want to roll all the way into the place where they can stay in the ECV the whole time, the 1400 way is a means of doing that.
Bama Ed