The best example is Baby Love. The girl's been in love with the guy (forgot the names) since she was 6 and he was 12 (I think).
I'm not sure if Dear Mine could count since
Spoiler (highlight to view)
satoko was pretty young herself (6) when Fuuma who was 2 fell in love with her.
Kimi wa boku no taiyou da by Toujou kazumi. I've only read Chinese scanlations so I apologize for just using pronouns. An 18-year-old college student takes in her older brother's 3 children after his death and their mom left. The oldest is a 10-year old boy who had been taking care of his two younger brothers. The 10-year-old ends up having a crush on the girl. Not incest :
Spoiler (highlight to view)
he was adopted by her older brother when his parents, friends of her brother, died
Both try to take care of the younger kids by themselves.Its a two volume manga. Volume 1 revolves around the kids moving in, the boy's crush, and the girl's relationship with her boyfriend. Volume 2 is set 4 years later, and is about their growing feelings for each other. Both try dating other people.
Any recommendations. No smut please.