This question is good for a critical thinking debate. You can tackle it from many stances. Let me try to share a few without going too deep.
1. Theology - The bible did say that he created man in his own image so adam should not have a belly botton if god does not have a belly botton. Now if god has a belly button, it means that he has a creator and this would defeats his alpha, omega stance. Now imagine this, adam and eve not having a belly button and their kids and grandkids be questioning them to why they look like alien without a belly button, this would be fun for them in trying to answer this. What if god already knows and because adam and eve will bear child since he did told them to go forth and multiply, god will give them a belly button anyway. Maybe a false one so they may look like their offspring. How about the stance that they were made perfect but after the fall, it is only then that they have a sense of awareness like right and wrong, belly button or not, vagina penis so if it altered their eyes so they can now see, their belly button also magically grew?
maybe god has no belly button and to enter heaven, you have no belly button cuz you don't need it in heaven. A belly button is a testimony that you came from man and through men after the fall from adam will also result in the sins of the father so all that are born thereafter inherited this sin thus why you see them Christians be baptizing to ward off this sin. Jesus Christ was baptized too so therefore, he too has a belly button, he came from his mom, Mary.
Perhaps adam and eve do have belly button if we deduce that they were not the first humans that the bible speaks off. There were more people roaming the earth but adam and eve were selected to tell a story? If they were the first 2 and their first born was cain, who did cain marry?
Unless he has relations with his mom or have relations with a monkey?
2. Darwin and the theory of evolution - If monkey have belly button and man came from monkey therefore, adam and eve have belly button.
3. ART - Well a good way to see the bible is through art. Artists drew adam and eve without belly button for a reason. They tend to hide it with a leaf that is so big that it covers their genitals and the belly button as well. The real deal is that no one knows what adam and eve really looks like so artist's rendition is up to his own imagination. Plus many Christians are illiterate and poor so the bible is taught to them by pictorial visual that speaks a 1000s words. It may not be accurate though. You know that statue of david with the penis hanging out and all, well his pisser is one good looking uncircumcised pisser but get this, david is a jew so jewish custom should be circumcise right?
I can go on and on with other aspects but the new year is here and the new me don't want to waste my time in trying to educate the few. Thanks for reading my junk by the way.