I tried a crayon quilt once. It took a *lot* of effort to lay the color down as a solid -- lots of coloring work. When heatset, I seem to remember that the colors faded a lot; basically could only get pastels or slightly darker. I thought it was a lot of effort for the result. I didn't use medium or anything; just used regular Crayola crayons. I tried an off-brand first, but found that those colors were much lighter and less effective.
How do these hold up to multiple washings? I think they would be likely to fade easily. Might be better for a wallhanging than a to-be-much-used baby quilt.
In terms of transfering the design, I don't remember now how I did it. However, I think if you trace the design onto freezer paper with a regular (not thin) Sharpie black marker, then back the fabric with the freezer paper, you should be able to see the design if you use a light box. Alternatively, you could use a light box to trace the design lightly in pencil on the front of the fabric.
IMO, you really need to iron freezer paper to the back of the fabric before coloring. Otherwise the fabric will stretch along the bias when you are trying to color in a solid area.