Dress up is always a hit at our house. We have a basic batman and superman t-shirt that we use for Pajamas. This is an easy, no-hem cape.
It is lightweight, thin cotton fabric and easily removable from a sleeping child so they don't get tangled up in it.
Pink the edges of what you will use for a cape. I measured the shoulders and then cut to the bottom at the wideth of the fabric, in a triangle, with a slightly curved bottom cut. If you fold your fabric in half lengthwise to cut it at least both sides will be the same...
Sew Velcro (prickly side on the cape) on each top corner.