Make sure you and your boys know how to use that bear spray, and practice using it before going, if youíre able.
As far as flies, my #1 go to fly all summer in the Yellowstone area is a size #16 Parachute Adams. I donít get over to the Cody area much these days, but Iíve spent the past 18 summers on the Yellowstone a few miles north of the Park, and fish many of the regionís waters from May to October. All of the other flies already mentioned are also good suggestions and will work well. Iíd also want to have some rusty spinner patterns.