Honestly what you have is perfect. Angles you don’t really have to worry about for the playhouse you listed on walmart. I watched a video on the playhouse and it looks pretty easy to put together. From what the video showed, it looks like the windows all around the playhouse are rectangles without the plastic trim around them. What i would do, is scrap the plastic trim for the windows, and make real functioning windows that can open and close. This would be easiest if you buy some plywood, measure how big the opening for the window is and cut it out of plywood, build a frame around the outside of the piece of plywood you cut probably with 1x2s and add some hinges and a lock. I would then add some hardware cloth on the inside of the window.
The video didn’t show the roof but it showed that it comes separate from the structure of the playhouse which is good because you can add some 4x4s or 2x4s so you get that upper air ventilation