Hmmm. I don't know if it's wheelchair accessible but if your party likes Seafood, then try Ivar's
on the Seattle waterfront. This is the sit down restaurant, and not the walk up window that is located next door. I've been to that walk up location, and it has great fish and chips, among other things.
Having grown up in the Seattle area, their Seafood Bars, as they're called, was one of our favorite places to go to for a casual fast food type of lunch. Their Acres of Clams location, though, is a sit down type of restaurant. Being you have such a big party, if you do choose this place, you may want to see if you can make a reservation.
I haven't personally been to this specific location, only their fish and seafood bar next door, which is a walk up window with an indoor type patio seating, overlooking the Seattle waterfront.