When doing this with younger students I've melted the crayons into muffin
tins and had them paint in groups around the muffin tin. They painted with
Q-tips... and when finished, some of them looked rather like pointilism. If
you're doing this with high schoolers, that technique make work really well.
I've also taken in a plug-in griddle and heated it up med-low. The paper goes
right on top and the crayons are used...they melt and blend into each-other
on top of the heated plate.
Also taking old irons, paint onto the bottom surface of the iron with the
crayon, then "iron" on the drawing on top of slick paper... ordinary paper
doesn't work well... you need to have a slick-surfaced paper.