MeruPuri: Märchen☆Prince, The Bride of the Water God Recommendations
They are both shoujo and the mangas have a prince in them that turns into an adult from being a child at special occasions.
A wonderful art with a fairytale plot. While in Merupuri, the main guy is a child and turns into an adult in the dark, in bride of the water god, the main guy is an adult that turns into a child during the day, both of them because of curses. In both the main girl marries the guy against her will but starts falling in love with him. Also, the main guys are royalties from another world, in Merupuri, Aram is from Astale magical Kingdom and in BWG, Habaek is from the Water Kingdom, while the girls belong to the human world.