I had a great experience with an arborist friend's 3 year-old twin boys. I was visiting his place and climbing in a small tree in his front yard to show him some "newer" climbing techniques. At the time (2006-ish) I was 51, he was well over 60, and climbing old-school. Tautline Hitch, Weaver saddle with spreader bar etc he dug my setup but wasn't going to change

No need to.
While I set my line and climbed both boys, one after the other, bear-hugged the trunk and started climbing below me. They made progress! It was amazing. I dialed them back as needed. I think there is a developmental stage for approximately 3-4 year-olds where the inner primate brain and body kicks in. They want to climb trees and they can. Especially with an adult in the tree above. I can imagine them looking up and thinking "You can do that? Me too". Imitation is the most fundamental learning process for ALL ages. Parents know that young kids learn from what they see astoundingly fast.
I had XS New Tribe saddles with me and put both on rope at the same time Blake's Hitch. It was super easy to position their ropes to hang on each side of the roughly 16' diameter trunk. No point in attempting to teach them to operate the Blake's. Most kids need to be at least 6 years-old+ to even consider showing them how to move up on a Blake's.
I was positioned center on the trunk and it was easy to alternately operate their Blakes and advance them by footlocking the tail of their lines (one at a time) and advance their hitches. Or simply pull the line down through their hitches and push the hitch up, they didn't weigh much. Going down was easier, I could've done it in a fancy way with a tethered accessory carabiner above their hitches but it was "keep it simple" easy to alternately move one down 4', leave him and go to the other, do the same. Their Blake's were out of reach of their dinky 3 year-old arms so very safe to "leave 'em and forget 'em for 30 seconds ;-) They liked that, they could wiggle-waggle or look around all they wanted during their mini hang times.
But seriously, I was in relaxed alert mode the whole time. Like always in tree climbing, precious life on a line.
-AJ