The recipe can be doubled in each ziplock as needed. The bag is zipped up, and put into another ziplock. The bag is put in to a large plastic bag, grocery bags work fine. This bag is put into a second large bag. Now, ice is added into the bag, covering the smaller bags containing the milk and sugar mix.
Salt is added to the ice.
This makes it colder and keeps it cold. Use a generous amount of salt. The bag is tied tightly closed. Wrap a towel around the bag and hold closed. Your child can help to shake the towel with the bags inside.