I've been meaning to try making some gel packs for the Man/lings in case they got bruised and bumped, but it just so happened this morning the Munchkin slipped down the deck steps (again). So Manling #3 and I did a quick search for solutions.
Apparently all they consist of is:
1 cup water
1/2 cup rubbing alcohol
freezer bags--1 for the pack, 1 to layer over top while freezing to prevent leakage.
You can double or triple the recipe depending on the size you want to make.
If you add more rubbing alcohol, the pack will be softer; easier on worse injuries.
No reason not to have some of these on hand!