Hogosha na Maou to Kodomo Yuusha

The Demon King, who was defeated by a hero 20 years ago, has been reincarnated as an ordinary young man and is enjoying life in this world. However, his peaceful daily life is shattered by the visit of a small child wearing a pot on his head and carrying a sword. The sword is the same as the one worn by the hero who once killed the Demon King, and the Demon King realizes that the hero has been reincarnated in the present day as a child. Even though he was once killed, he cannot leave the young hero alone in the present day...

(Source: Natalie, translated)