Save Naked or Odwalla juice bottles and fill with water and freeze. These are just the right size for a child's lunch box. When they thaw, you can have cold water to drink. These are also useful for placing in a larger cooler.
Why spend money on ice keepers for your lunchbox when you have a plastic lemon squeeze bottle that does the trick. Fill your lemon squeeze bottle 3/4ths with water and freeze. The bright color stands out in your freezer and you can have your kids pack their own lunches easily too. The lid stays tight and you're on your way.