A Gent's saw is not going to fit in those window openings. Cut them with a trim router and bushing, then square the corners with a file.
I built a similar scale horse barn 14" X 36" X 12" high from 1/2" BB for my grand daughter a few years ago. It had to be sized to be in proportion to the plastic horses that she already had. For the windows and door openings I used my laminate trimming router with a 1/8" spiral bit. a template, and a 1/4" router bushing to cut all of the openings. I used a file to square the corners of the openings and then made L shaped molding and cut them to make mitered window and door frames for the openings, which were glued in place. To simulate a metal roof, I cut parallel slots in the roof panels just deep enough enough to cut through the top veneer of the plywood using my Unisaw, spaced 1/2" apart. The roofs were designed to lift off, with strips glued underneath to catch the top edges of the walls to keep them in place. I made the doors and window opening covers (horse barns don't have glass windows, just short doors in the window openings) from 1/4" BB and glued mitered 1/16" strips on the face side of each door, complete with the traditional diagonal brace for a simulated authentic look. All the doors and window covers were hinged and latched with the smallest brass hardware that I could find. She and her mom painted it red with white trim and a silver roof, and then glazed it to make it look aged. The result was great. Unfortunately, I lost all of the build and completed photos in a computer crash a short time later. Some day (soon I hope) they are supposed to visit and bring the barn back so I can take some new photographs of it. They will be posted as soon as I can.
Central North Carolina