How old/big are the boys? How soon until they start hunting elk? Are they hunting/shooting now, and if so what calibers? You could always start them off with something smaller, and move up.
Like say 25-35AI bbl for deer, smaller game, then add a larger barrel as they get more proficient. I can't see you needing anything larger than a 30-30AI or 309 JDJ with a 165 to 180 gr bullet for elk.
My buddy's daughter just flattened a cow elk this week with my 308 Rem 700 and a 165 partition. That's close enough to what we're comparing to. She's not a young kid, 15-16 yrs old, and been shooting and hunting for a few years now.