Hi FH,
The KRT with a 4 year old -- I'd ask if the kid is used to hiking something that steep. Alex at 4 could not have handled it, and the two of us together barely managed it when she was 5. The KRT from the tram parking lot is a horribly steep, relentless slog. Alex and I were not fond of it At All. If and only if they are willing to carry their child when/if she tires would I say go for it. If they are expecting her to do the whole thing on her own, then they all might be in for a very grumpy and sloooow ascent (if they make it at all). It's a difficult trail, not one of the "easier" ones by any stretch of the imagination. In retrospect, I wish I had held off on the trail til Alex was a few months older and more experienced. It was the fourth one we did -- and did I mention we weren't fond of it? lol