I have had used three different Kelty models (Pathfinder, Journey, Wallaby). All of them were great. I carried my son 10 miles (~22 pounds at the time) in one day with the Pathfinder. At the end I was tired and a bit sore but nothing major.

The Wallaby is great for infants but not something I would take into the backcountry. My suggestion is to take you kid to the store with you and try them on with him in the carrier. If they won't let you do that, go to the next store.