These are super easy to whip up, whether you use a sewing machine or hot glue gun.
I cut 4 different colors of felt into 7" wide by 12" long rectangles. Then I angled one end to make it look like an envelope flap.
The button was to go on the end that still had square corners, about two inches down from the edge and centered. She chose her favorite buttons from my little jar and got to stitching. She was super careful and made sure the needle went back in the same side it came out...great progress from her first lesson!
Next I folded the end (with the button) up a little more than a third and stitched the side edges together. Hot glue would work great too, or a young seamstress could even hand stitch those seams.
Then I folded the Flap side down and cut a slit to fit over the button.
Finally, we cut out little felt Stamps and hot glued/stitched those on the front of our envelopes.
They've had fun writing notes and delivering mail to each other. Best perk of this project: my sweet little love notes!