Candle supply places do sell glitter specialized for candle making but it’s usually made of either glass or metal——definitely not advised for working with children.
The main reason is because regular glitter is made from a shiny polyacrylic material which only looks metallic but really isn’t. If you were to mix this into the wax before pouring over the ice cubes it would be directly affected by the flame thus creating the potential for toxic fumes.
It might be a little more difficult than coating a solid candle but you could try painting it on the outsede once the candles are completely hard and removed from the milk carton molds. Do a few tester ones beforehand to see if you like the results.