Reporting on the first success I had with an autoinflatable. It took a few attempts to get it right.
I got a good reaction using 2 teaspoons each of baking soda and citric acid. Plus a few shakes of the glitter containers for good measure. 
I heat-sealed three sides of my shrink wrap, then added my ingredients for the chemical reaction. Last, I filled a tiny ziplock baggie with about 10mL water. I put that in the shrink wrap bag and heat sealed the fourth side. I taped around the edges to make sure it was air-tight.
Here’s the before picture.
And here’s the after picture.
And here’s the autoinflatable in action.
The first few times I tried this, I didn’t get much inflating action. It may have been because I didn’t have a good proportion of powdered ingredients to water. But with this last attempt, I was really happy with how quickly it inflated once we gave the inflatable a bit of a shake to mix up the contents. I have to admit, though, the glitter effect was not as swirlingly magical as I’d hoped! 
If I did this again, I’d forgo the shrink wrap and just use regular mylar because it’s much easier to get a good seal with mylar.