Unfortunately i disagree most humbly with the other posters on this question.
Yes, i do believe both Adam and Eve had belly buttons. Why? Because scripture says that Man was created in the image of G-d. Meaning that G-d gave Man everything He had Himself meaning. God has canine teeth(He eats meat), a penile foreskin, and a belly button.
How come G-d has a belly button? From out of the Chaos that He came from, formed his own belly button.