I bought a jigsaw piece silicone mould off Ebay and then myself and Jack filled the cavities with broken pieces of crayon.
We then baked them in the oven at 140°C for 10-15 minutes until the crayons had melted.
Once the crayons had set (with the help of a few minutes in the freezer because we're impatient) I turned them out and Jack tried them out
We made some single coloured crayons and mixed others to make multicoloured crayons. They would be great as party bag fillers and you can get silicone moulds in lots of different designs - I particularly like the Lego men crayons here.