I got this from startsampling.com, of all places.  It seems like it would be fun and easy for kids.

Homemade ice cream

Here is a quick recipe to keep the kids busy and provide them a delicious treat!

1/2 cup milk
1/4 cup half and half
1 tablespoon sugar
1/4 teaspoon vanilla extract
2 sandwich size Ziploc bags
1 one gallon size Ziploc bag
2 cups ice
1 tablespoon coarse salt

1. Put one small Ziploc bag inside the other then add the milk, half and half, sugar, and vanilla extract to the inside bag.

2. Remove excess air & seal both bags.

3. Add the ice and salt to larger bag and place smaller bag inside. Seal firmly.

4. Now it\'s time for the kids to shake, rattle & roll the baggie. Ice cream will be ready to eat in 5-10 minutes.