In my book, one of the elements of a great craft is that it is easy to do. Especially, actually imperatively if you are doing it with kids. This one meets that criteria. All you need is some crayons, a pencil sharpener, some wax paper and an iron.
Choose the colors you want to use in your melted crayon craft. I find it looks better to stick to a limited, complimentary color palette, but most kids just want to use every color you have out. You decide.
Remove the paper wrapping from the crayon, and sharpen it until you have a nice pile of crayon shavings. (You must use a small, hand held pencil sharpener for this. NOT electric!)
Heat your iron to low.
Lay an old towel or paper bag on your ironing board or kitchen table. Lay a longish piece of wax paper on the towel/bag. Let you child sprinkle crayon shavings all over the wax paper.
Lay another piece of wax paper over the top of the shavings and run your iron over the wax paper.
If you are concerned about getting melted crayon on your iron, lay another bag on top of the wax paper and iron through it. This will not allow you to see the crayons melting which is one of the best parts. What I do is use the cheap iron I got at Target for $10. It is reserved just for this craft. You can also find irons at thrift stores or garage sales. Believe me, it is a worthwhile investment.
The paper dries almost instantly. Your child can then cut the wax paper into shapes to be used in all different sorts of ways. Hang them from the window like stained glass ornaments. Glue to paper and make a card. Frame it in construction paper for a real stained glass window look. String together to make a garland.