That is such a fun project.
my oldest loves to "melt stuff" and she has melted so many crayons. The plastic trays that are for candy making work great(you have to pour the melted crayon into it) and they pop out so easy. I picked up one of the silicone trays and that didn't work too good. The wax stuck to it and the new crayon didn't have a smooth surface. Not sure how to really explain it.