I bought a 20 pack of light blue ponchos off Amazon for our trip.
For your six year old, sacrifice one of the ponchos and see how it fits on him before you go. If its too big, and you cant rip off some of the plastic, buy a bunch of smaller sized ponchos for him.
Try the camping section at Walmart. Or the camping at Menards. I just went looking for these for an upcoming Boy Scout trip and found them around a dollar at both. I think Target had them with camping gear as well. Definitely go to the camping section.
I looked all over our Wal-Mart for them, I figured they would be with the umbrellas and raincoats. Nope, I finally found them with the camping gear they were about $1.50ish. They were slightly thicker than the yellow ones at Dollar Tree.
Thanks for the advice! I ended up finding them at Walmart in the camping area. Never thought to look there! I was just looking with the umbrellas and travel stuff.