>How did they train for the hike? Is it a matter of just being active kids?
My datapoints are a daughter (7) and a son (12).
Daughter (7) is a stud muffin. The world is her gymnasium. She doesn't walk down the stairs - she climbs down the bannister. She also has always had a strength-to-weight from where/who, we don't know - iron cross on the rings, can climb a vertical door jam with finger strength, etc. For her 7th B-day party, she requested a rock climbing theme so I built two 24-foot-tall climbing walls. I was leary of her on Half Dome last week and the deal with my wife was that they'd turn back when half-baked. But the deal with the daughter was, "If you want to climb one really big rock today, you can't get distracted and climb all the little rocks along the way. Them's the rules today!" She made it, and the cables were a big, positive kick-in-butt. She was tired for the last half of the downhill and there was more literal and verbal handholding to keep her moving.
Son (12) is all slow-twitch muscles like me - not a sprinter AT ALL but we can just keep going all day long. He was 10 when I realized he wasn't slowing me down on day hikes (I carried the daypack). For 6 weeks before the Grand Canyon (at age 11), we'd each spend 10 minutes most mornings doing stairs in the house - up & down, up & down - get sweaty, and then shower and start my day. That, and we did flat miles (the only kind around). 2 to 5 miles on a beach or trail a few times a week while talking about life, math, Harry Potter, whatever. Last week up Half Dome, we'd prep'd much less. He did great, never slowed us down or complained about anything. He was only a little sore. I (51) was moderately sore 2-3 days out and now, 5 days later have no issues. If he'd done some prep work (we both will next time - Whitney!), I could put some weight on his back (and off of mine).
Neither of them walk to school like I did. We make them get out of the house at least once every day or they'd read non-stop. They climb trees, make snow forts and sled down hills. He (12) has PE in middle school. School has grade school recess outside down to -10F. At home we kick them outside to -15F. Family activites tend to be active - hike, ski, canoe together a few times a month. For years, we'd tried to keep the backpacking and hiking fun rather than impressive but now we're including some impressive, iconic stuff as well.