What is this thumbprint sized dent above my belly button & how can I fix it? (photos)

Its really hard to tell from the photo, but you could have lost fat there, or there could be an issue with the body wall....that's a good one for a physical exam.  You might be able to excise some skin, or perhaps graft some fat.

An examination is going to give you the best answer and treatment options if any.  Schedule a consult with a board certified plastic surgeon.Earl Stephenson, Jr, MD, DDS, FACS

It looks like you have some irregularity of the fat in that area. I would need to examine you to be sure.Could probably do some liposuction around the indented area, and possibly some fat grafting to the depressed area.

Irregular fat deposits. That can be treated by directed lipo of fat grafts in the valley area. Or both options..... 

It is impossible to adequately tell you what is the cause of the indentation without examining you.I
would suggest you find a plastic surgeon certified by the American Board of
Plastic Surgery and ideally a member of the American Society for Aesthetic
Plastic Surgery that you trust and are comfortable with.  You should
discuss your concerns with that surgeon in person.

Robert Singer, MD FACS

La Jolla, California

Difficult to tell without an examination. Please feel free to give us a call and be seen. There will be several options. 

You will need a formal examination by a board certified plastic surgeon for a diagnosis of the cause of the didn't above your belly button.  Since it occurred after childbirth it may well be a small split or separation in the rectus muscle directly above the umbilicus.  Please consult a board certified plastic surgeon.