Another option for taking the little-little ones on the trail are backpack-style child carriers. I had a MEC Happytrails Child Carrier when my kids were little and thought it was great:



It has an 18 litre pack that connects to the frame under the child too so you can still take some gear, food, etc. with you.

The big advantage to this is that you are only limited to terrain you could otherwise hike. For full-out hiking, I think these are the best option.