Loch Chiarain?
If you did it from Kinlochleven up the WHW then it is a big landrover track climb, an exciting crossing of the reservoir, a boggy flat 2km then some lovely singletrack to the door of the bothy (mostly uphill).
Next day you can return the same way.
However if you approached from Kinlochleven up the Mamore Lodge track, across the outflow of Loch Eilde Mor and then following the obvious track up and around the hills to the bothy, it’s one big long descent to the bothy on some really awesome singletrack. Next day you could return the same way for more descending.
I’m trying to think of your 13 year old when I type this, although I’m not sure how long the climbs would be on a bike, I hiked them.
Edit: Maybe verging on too big a ride to be honest, maybe check the distances!