Remove them carefully with hot pads. (Obviously, this is a grownup step.) Test your rocks to make sure they're not to hot for a kiddo to handle. Ours felt just about perfect for a hot stone massage (hmmm -- how to train the kids to do that??)
I placed each one on an old kitchen towel (don't use one you care deeply about, as I'm not sure if the crayon will stain it), to make it easier for the kids to handle.
And just let them color away!
We've done other explorations with melting crayons (melting the old ones in a silicone muffin tin), so the kids were curious about how we were going to melt these. They thought it was pretty fascinating to see them melt on the hot rocks!
If your rocks get too cool, you can put them back in the oven (place them on a cookie sheet first, so you don't have to handle them after the was remelts a bit) or in a microwave (place on a microwave-safe plate for about 10 seconds.)
We decided these would be cool paperweights or garden stones -- what do you think?