MeruPuri: Märchen☆Prince, Lunatic Honey Recommendations
they both have a girl who bumps into a tiny child, and then letting the kid stay with her in her house. when the boy stays with the girl, well, they share a bath. they also both have a guy who becomes older. and of course they both have romance
The female lead takes in a cute little boys and in both manga's he transforms into man overnight.