|Unfortunately I haven't read the given examples, but maybe you can try Tenjou Tenge, because one of the main characters is (at least in the Anime) able to change into a childlike form, or The Bride of the Water God, if you don't mind Manhwa? ^^"
Spoiler (mouse over to view)
The storyline gets by time messy, because of the many twists and characters involved, but the male main character changes his form, too. By day he is cursed to look like a child (Habaek) with reduced powers and by night he gets back to his adultlike form (Mui). Also, the female main character is in the beginning unaware of those changes.
"It's like love is a bonus in your life. Even without it, I'm just fine. I don't feel like pointlessly searching for it. You don't have to make yourself unhappy." (Ookami Shoujo to Kuro Ouji)
"Why does it rain?" - "To create drama." (Drops (Andou Yuki))