Is this going to be a wall hanging or a quilt?
If it is something that is going to be repeatitively washed, I would go with fabric markers or pigma pens and heat set. If it is a wall hanging, than crayons would be a great option.
A suggestion is to iron the fabric onto freezer paper either way, to make it easier to color on and put a frame on the right side of the fabric with blue painters tape, to keep everyone well within the seam lines :wink: