The ziploc method works very well. Here's another verison that also works well:
Replace the outer ziploc bag with a #10 coffee can with lid. Put the ingredients inside a ziploc. Close and put the ziploc inside the can. Add ice and salt. Put on the lid and have the kids roll the can back and forth between them on the ground. If you have very active kids or bumpy ground, you may want to duct tape the cover on the can. Depending on how warm it is outside and how much ice and salt you use, you may have to open the can and add more of each.
Growing up in my family, making homemade ice cream with a handcrank maker was a monthly occurance. More often if we had company. We all had good arm muscles

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With our kids, my husband and I have gone to using an electric maker, but we do have a handcranked one and use it occasionally for youth activities and such. We've even used it at a teardrop gathering where we made dutch oven peach cobbler over the fire. We had a lot of happy campers!
Betsey