Wait, which is the secret behind "childhood friend"?
There is and isn't a secret. They are really childhood friends, but due to the cultural difference, they also are "family like". Mostly because you past most of your free time with her/him, whatever in her/him house or yours. Generally in doujins you can hear the MC's mother say that the childhood friend is the daughter she haven't had, or that she's like a daughter for her.
But I'm not sure that it's a secret code to hide incest, since Japan have "official" mangas like
Super Taboo, which are full incest. For me, it's just because childhood friend is something really frequent and natural in Japan. Add to this the fact that they aren't related, while growing together, and it's also natural that one's first crush is the said childhood friend.