=/ ~throws bible in a fire~ lol jk.
I wouldn't know what to tell you. Fictional or real, since I don't care for religious ideas such as this, I would believe they never had belly buttons.
From an artist standpoint, both Adam and Eve had belly buttons since they are the first two to be born on this earth and must have been made as such to continue our generation to have belly buttons as well.
If you think about it, if they didn't have belly buttons, then we wouldn't either since we'd be born the same way. Though this is just speculation from someone that doesn't care for religion. Also because of their form of birth, they changed reproduction of the human species from forming dirt and a male's rib to sexual intercourse and giving birth from the woman. So in some ways they did since we have belly buttons, and art is actually one of the few ways to show they did. They are representation of male and female, the first of our kind. Of course they should have it since we do as well, no matter the process of birth.
Iunno, this is just another question like, "What came first, the chicken or the egg?".