Thanks for the assistance, guys. Because I let this go down to the wire, I didn't have time to order any of the excellent suggestions or materials you guys mentioned. But, your ideas got my creativity going. I ended up going around looking for yoga mats! I found a couple at Walmart, and I'm going to try cutting them out to make the front and back portions of the vests. I then have velcro that I'll attach to them to finish off the job.

The problem I'm running into now is that I can't think of a place to get a realistic gun. Understandably, it's difficult to find a toy gun that isn't extremely bright and cartoony. Thanks to the wonders of Wikipedia, I've found that Beckett carries a Glock 17, but I'll be happy with anything approximating that...