I've been climbing with my 4 year old quite a bit, he's good at climbing limby stuff, otherwise it is just hauling a sack of potatoes up. Once we get to a good spot he loves to sit on Dad's lap and hang out. We were in a beautiful Ostrya virginiana today, checking out the funky bark and rows of holes from sapsuckers. So cool.
My 8 year old niece put on my boy's harness and gave it a try. I was blown away by the difference a few years makes, strength and coordination. She was pulling herself up no problem, and was even tending her own slack on the blakes' hitch, just from watching how I was doing it for her. I can't even imagine how good my boys are going to be in several years, giving the squirrels a run for their money I suppose.
My 2 year old gets consistently mad at getting left behind. My wife made him happy by holding him and swinging a bit a couple feet off the ground.
One big thing to consider is kid's lack of understanding of what is and isn't a good idea. I've explained to my boys that tree climbing is only allowed when Dad's there, but listening to Dad isn't as important as I'd like it to be. The boys are crazy about tree gear and ropes, but their good at imagining things too. My wife busted the oldest one midway up a 15' spruce trying to "tie-in" with the dog leash, a puppy harness and a squirt gun in his shorts, for "pruning" I suppose. I had a good laugh, and then thought, have I created a monster?