Oya No Kao Ga Mite Mitai
A lawyer is dumped by his unfaithful wife in a gay cruising district and has a one-night stand with a guy who turns out to be his son's kindergarten teacher. Taiga, the man's son, is one of the most charming child characters I've encountered in manga. He openly declares his love for sensei, going so far as to loan him to his father until he's ten years older.
Ikumen After
This is The Happiest Story. Two single dads meet in the mornings dropping their sons off at school. The sons are dear friends, and the dads grow close, helping one another to cook and manage their lives. When one loses his apartment in a fire, they combine households temporarily out of necessity, and it becomes harder to hide feelings for one another. The dads are great, but the kids MAKE this story. They're adorable. Be aware that Kentarou works in porn at one point, and having sex with someone else is part of that work. He sees it as work and so does Asakura.
Hanakoi Tsurane