I had a Little Red Riding Hood costume when I was small and then passed on to my DDs. It was only one layer of fabric, and served well. The fabric reminded me of sail cloth but was very soft. Many washings maybe. WE all carried a basket for the candy and wore one of our own dresses to complete the costume. I don't think you need interfacing--it will make the cape stiff. Your little one is going to look so cute.
Sue