Be sure you get all the salt off the small bag before you open it. Stir in sugar and canned milk. Let out as much air as possible, and seal. If too much of the ice turns to water, empty water and add some more ice and salt. You don't need rock salt.
Brownies can help measure, mix and take turns shaking.
I have done this recipe with every age level, and it works well consistently. Girls can eat out of the individual bags, so there is little clean-up necessary. Just the original mixing bowl, spoon, and teaspoons.