I feel that the myth of "old people becoming onii (demons) and eating their children" is a poorly made excuse. In ancient times in Japan, most ordinary people were very poor and their lives depended heavily on farming. Furthermore, farming techniques at that time were quite backward, so the yield was not high, or to put it bluntly, very low. Not only was the output low, but they also had to pay taxes or pay rice to the court, so the amount of rice they actually had was not much. Therefore, the workforce of adults who were able to produce food was the most important. And of course, who were left behind? The old people were no longer able to work, they could only consume food but could not go to work. So they were considered to be a "burden" to their children. Eventually, the old people will be abandoned by their own children and starved to death so that no food is wasted feeding them. And yeah, their children made up this story to erase the guilt of abandoning their parents.
If you want to experience the world of Kihachiro Kawamoto, come to Iida city (Nagano prefecture in Japan). There is a museum that is established for the dolls of Kawamoto. Iida city is the most famous city of doll theater in Japan.