I agree with Lori's approach. However, you could, if so inclined, buy a couple of yards of a large Avenger print fabric, border it with a fabric from your stash that coordinates with it, use muslin backing and quilt it. The sandwiching, quilting and binding would take the most time. This approach minimizes the time invested by you in making it.
I made Pokemon pillow cases for my 'grands' and they were absolutely thrilled to get them. (Pokemon fabric is impossible to find, so that's all I could make them!) My guess is these two little boys would be just as thrilled with Avenger pillowcases, and pillow cases are much faster and less labor intensive than a quilt.