We made pinwheels at the playground.
We used an old butterfly calendar to make the pinwheels. If you can find a piece of paper with color on both sides, all the better.
My friend folded the paper diagonally to check for squareness. It turns out it that the calendar was perfectly square.
She then folded it diagonally the other way.
She cut along the four fold lines to within about an inch of the center. She now had four triangles joined at the middle.
She bent in the right hand corner of each triangle.
Here is what it looked like below.
She stuck a pin through the center of the pinwheel into a stick we found on the playground. That was very difficult to do and did not last very long because the pin kept falling out. Also, this size pinwheel is too weak and floppy to turn well.
I suggest that you make your square no bigger than 7 inches on each side.
And here is a better way to assemble the pinwheel. Stick the pin in the pinwheel into the end of a wine cork, and put the cork on the pointy end of a wooden skewer. How long the pinwheel lasts depends on the strength of the paper and the vigor of the play!
Here is a seven year old running joyfully all over the playground with a pinwheel.
Then this little three year old had a turn with a pinwheel. It was a windy day and the pinwheel turned merrily on its own. The little boy was so proud of it. He watched it for a long time.