Started both my kids at two or three, in a full body rock climbing harness. Now my older one has a new tribe. I just set them up DdRT, and help them on the ascent. My boy is eight now and the last few times we went he was ascending by himself.
The last time we climbed I set him up srwp with the bulldog bone, a foot ascender, the haas, and my swing chest harness. And he knocked it out of the fucking park. Ascended like twenty or so feet, then did a limbwalk out to the tips of a branch and came back. Absolutely killed it. One of my proudest moments as a father. He also started foot locking last year. It's a lot of fun. I will usually set up a foot locking line and hang them from it, then give em some pretty big swing action. That tends to be the big draw with the kids.