While I think you are on the right track with the interfacing, is it really going to make a difference for a 1 yr old's costume? How much is she going to be standing/walking that the drape is going to be noticed? If she's going to sitting up mostly in the costume, you might want to think about using a stiffer interfacing to give it more of a shaped appearance? We don't have Hancock's here but I know JoAnn's carries a ton of different interfacings. Sorry I can't help you with specifics on the various types as I've not done any garment sewing in decades but I'm sure the folks in the store can help. Or...check out the pattern section of either store. Look at a cape pattern and on the back of the package for supplies it should give you information as to type of interfacing needed for that particular pattern.