Caution: Please keep an eye on young children and limit their time outside when it is very cold. Children sometimes do not realize that they may be in danger of frostbite or hypothermia.
Next, put a couple grains of sand into each balloon. The sand will act as a freezing nuclei for the water. Without it, some of the water in the balloons may not freeze. To learn more about this effect, look up "superchilled", "super cold", or "super freezing".