Sodatechi Maou!

Death Castle, the seat of power in the Demon World, is home to three demon kings...and a toddler?! The High Demon King, Death Mask, and his co-rulers Demon King Giant Rock and Demon King Dark Knight find that though they command powerful legions of evil, they aren't immune to a child's cries...and so the sweet, innocent human child Maris will grow up under their care! After all, how hard can raising a child be...?

(Source: Kodansha USA)