crayons work great...after the artists are finished place a piece of a brown paper sack over the crayon-colored pictures (to absorb the wax) and heat set...the color will set the wax will come out on the brown paper.
i've been making coloring quilts with a local pre-school for 10 years now- they come out great and last...the oldest pre-school quilt we made still looks pretty good- although one of the blocks was (recolored) once- it was very light to start with...we just re-colored and re-set it...perfect.
for the adult blocks- if possible- i recommend MICRON PIGMA PENS- they come in many colors- are permenent- do not bleed- are made for writing on textiles- are acid free- do not fade- archival quality....will last- no skipping--easy to use.