This could be a first project, or a project you can do with your child. It only takes a short amount of time, and could be done with a standard home-grade sewing machine. A small amount of Sunbrella (or alternative), small zipper, and a bit of cord is all it takes.
The dimensions shown here are not critical at all, and in fact, you may wish to change the length, depending on the length of your flag pole. Once you have the Sunbrella cut out, begin by sewing a 1/4" or 1/2" hem all around the inside of the flag cover. You will then lay the 1/8" dia cord along the bottom, fold over 1", and sew a channel for the cord.
You will probably have better luck with the zipper installation if you fold the cover inside out. Just remember to re-fold the cover back before the last step.
Continue by installing a #5 YKK zipper, or any other zipper of your choice. Chances are that the zipper won't be the same length as the flag cover, so install the zipper from the lower edge. That will allow a "pocket" to be formed on the top edge, which will help in sliding the cover on. Alternatively, you can use Velcro. The choice is yours.
Finally fold the flag cover in half, then sew the top as well as any length down the side until you meet the zipper. Attach a barrel lock, tie the cord ends, and you are done!