I used to have a mine site that required monthly water quality sampling at an elevation of about 8,000 feet in central Nevada.
We used to drive to the end of the road in the dark, then ski downhill 2,000 feet into Birch Creek Valley, ski 8 miles downstream, 8 miles upstream and carry 30 pounds of water samples and our gear back up 2,000 feet hopefully by dark.
Fifteen miles on skis is an easy day on level to moderate topography with fast tracked snow. It would be a hard day in steep topography if you had to break trail, or have transitional snow. Fifteen miles would be ambitious for kids under any circumstances.
In the Olympics, races (sprints) are usually 10, 25, and 50 kilometers, 6, 15 and 30 miles.