Sure, frozen peas work well enough in a pinch,
but if you're prepared (and with a house full of boy scouts, it would be a good idea), a soft ice pack from the freezer is better. And one that just cost pennies (less than a bag of frozen peas) and uses common household materials would then be best!
Take two quart-sized freezer bags. (You can use snack bags and make them smaller and without extra plastic, but I like the thickness of the freezer bags which decreases the risk of getting a hole--you can choose. It might depend on how prickly your most-likely-to-be-injured-family-members are.) You may want to label the bags in some way. I just wrote on mine with a Sharpie.
Fill the unlabeled bag with 3 parts water and 1 part rubbing alcohol. (More alcohol will make a softer gel.) You can add a couple drops of food coloring if you like. This not only looks pretty, but it looks less like regular ice and can help prevent anyone forgetting that it is NOT edible. Seal.
To decrease the risk of a leak, put that bag inside the second, labeled bag and seal.
Place in freezer for a few hours or until needed. I made one with 1 1/2 cups water and 1/2 cup 70% Alcohol and one half as big--3/4 cups water and 1/4 cup alcohol. You can freeze them flat, or upright. See, can't you feel the chill? Good thing I froze them for the picture.
Ahh, now that's cool.
It works for injuries, coolers, and cuddling up with on a hot day! Then put it back in the freezer until you kneed it again.