Here is a very easy to construct Bat costume that would work for either kids of adults.
Take a long-sleeved black shirt and gather these measurements…
Then using those measurements, cut out a wing shape out of black fleece or felt. Cut out two layers of fabric for each wing. I did this by lining up one edge of my wing pattern on the fold of a piece of fabric.
Make sure one of your wings points to the left and the other to the right.
I sew the two layers together (right sides together), leaving a gap to turn the unit right-side out.
Lay the wing over the edges of your shirt and pin in place. Make sure you tuck the seam allowances of the gap inside the wing. Sew all around the wing, attaching the unit to the shirt and closing the gap in the wing.
Then add some decorative quilting to the wing; two straight lines going on a diagonal to the points looks nice and very wing-like.
For a funny finishing touch, why not add a bat mask (sized for children)? You could also do ears on a headband or a stocking cap.
BTW this was my model, Libby, discovering her bat costume package in the mail. This expression is priceless and great motivation for making costumes!