When I was on an assignment stationed in Japan, I saw this show, and I could not figure out how they do Chiko-chan's head and face. What little I could find indicates that it is some combination of a person in a suit and CGI, but I haven't found any behind-the-scenes images that show what exactly goes on.
My guess is that the suit doesn't have the head as you see her, but there is some kind of motion capture tracking stuff there. You can see that the shoulders of the suit seem to move through where the head is, so what is physically there must be smaller. The other people on the show seem to react to her in real time, so they are probably seeing facial expressions from the actor visually.