And depending on how many parties may be in need of that ONE accessible boat, there may be a MUCH longer wait than in the mainstream queue. The OP may have gotten lucky and had nobody ahead of them in the wheelchair line, or the grandmother may not have needed to stay in the wheelchair to board the boat (or both). Many times, the regular line at Small World has a wait of five minutes or less - boats leave partially full, Guests aren't arriving at the loading area fast enough - but parties with wheelchairs have to wait... and wait... and wait... for the boat to circle around.
We have waited in that line for over 1/2 hour during a time when Small World wait for people walking in were basically loading as fast as they could load.
Peter Pans Flight does indeed have an accessible entrance, I have used it.
It's a Small World you do not "go Partially through the regular queue", you are directed to the exit and enter there.
Peter Pan has an accessible entrance, but that is only because there was no room to make space for guests with wheelchairs to wait in the regular line.
That does not mean getting on right away though. Guests may be boarded fairly quickly because there is not much space there for waiting in a wheelchair. They may also be given a handwritten Fastpass or advised to get a Fastpass.
This is what the official Guidebook for Guests with Disabilities says about Mobility Access for Peter Pan:
"Obtain a FASTPASS OR see a host for options. If FASTPASS is not available, see a host for options."
Because the moving walkway on Peter Pan can't be slowed or stopped, many people with disabilities are not able to ride it at all.
Small World is presently under renovation and I don't know what access will be like when it is finished. I have heard that when it was renovated last time, they did try to make a Mainstream queue, but the Fire Marshall would not agree to it because someone in a wheelchair could be trapped on the wrong side of the water during an evacuation. So, how it was before the renovation was guests with wheelchairs/ECV/mobility devices waited in the regular line until the point where it is no longer possible to cross over the water. At that point (about 2 turns of the queue), there is a pull off point for guests which leads to a ramped bridge that goes over the water. This ramped bridge also was the exit. If it is not busy, the CMs did sometimes direct guests with wheelchairs/ECVs immediately to that pull off.
I have read that the current Small World renovation will be to make Mainstream Access in a way that satisfies safety concerns. We will have to wait to see how it is once it opens.
The Studio and AK were built totally with Mainstream Access because of when the parks were built. Attractions at Epcot and MK were changed to Mainstream access as new attractions (like Mission Space and Soarin') were added or attractions (like Imagination) were renovated. There were some where it was not possible to convert to Mainstream access; those are the few remaining attractions that have a special handicapped entrance. Most of them are in MK because there was not space to make the attraction accessible.
I anyone wants information about travel to WDW with a wheelchair, ECV or disability, please visit the disABILITIES Board (there is a link in my signature and also in Cheshire Figment's).
There is also a link in my signature to the disABILITIES FAQs thread. Post # 11 of that thread lists the "Mobility Access" for each attraction from the official Guide for Guests with Disabilities for each park.
If you look at it, you will see that in most cases, the Guide does direct guests with disabilities to either get Fastpasses or use the regular queue. There are very few with any other direction.