What you do:
Put sugar, milk and vanilla in the small bag and seal it good and tight. Fill the big bag half way with ice and the salt. Put the small bag inside the big bag with the ice and salt. Shake and roll the bags around for about 5 minutes. Wipe the salt water of the top of the small bag and open carefully. Voila! Ice Cream
The ice and salt gets really cold, so give the kids oven mits or a towel to handle the bags.