Well, I guess I'm posting a trip report for my daughter, who is 5.5 years old. (And that last 0.5 is VERY important to her.) We took the cog railway up Pikes Peak and hiked down Barr Trail, a bit over 12 trail miles in 6hr 40min. I carried her in the baby backpack for two of those miles on the flats around Barr Camp, but she hiked at least 10 miles. I was pretty impressed.
Needless to say, she bounced back better that my wife and I did- after all of that downhill we limped around for a good 4 or 5 days afterwards. Stairs were hell.