https://www.fisherie.com/Maps This link will help you out a lot Dpm. I've got a 13 year old hard core fly fisherman, and a 7 year old bait chucker, so no matter the age of your boys, they'll have fun and make some memories.
As for a general starting point, I'd say pick any one of the parking areas marked with a "P" on the lower or upper Elk Creek maps. Because Elk is so big, and there are no major impediments to upstream movement, it's a great place to start out because you can generally find some room to get away from the crowds and you'll almost always find fish from the lake up to I-90 at this time of year. More rain in the next couple of weeks will move them even higher, if they're not there already.
If you don't mind fishing in heavy crowds, you'll find plenty of fish of lower Walnut as well. From the lake up the where it says "falls" on the map.
I don't fish many places other than Elk anymore in the last couple of years. Once in awhile, we'll squeeze in somewhere on Walnut. But from what I've read in the last week or two, nearly all of the streams on that map have fish in them now.