It's a good idea to make these up way ahead of time. We keep them available all the time, so anytime we have to run, we just grab a couple and go.
Start by freezing the plastic ice cubes.
While those are on their way to freezing, fill a large gallon size ziptop bag with dish soap or after sun aloe gel. These items are usually chosen for color. You can also use shaving cream, pudding, jello, etc. or anything else that you can think of.
Seal up the top of the bag and refrigerate until cold.
When you are ready to head out, pull out the bags you want, and fill each with a handful of the frozen plastic ice cubes. Reseal the bag. If you are worried about a child opening the bag and everything spilling out, go ahead and tape it shut. You can freeze the whole thing after that, and use it as it's thawing.
Now, you have a cold pack. We make these up for our really little ones mostly, though trust me, I use it too. It's a fun texture, it looks super cool and most importantly, it stays cold for awhile.
You can lay it on your stomach over the top of your clothes, hold it on the back of your neck or even on top of your head, which my kids find hilarious to do. A very young child, can simple use it to play with in their hands, achieving the cool down effect that way.
Side note: If you decide to freeze the entire bag, you will always have an ice pack when there is a bump or bruise. A benefit too!