The photo shows that you have several findings which would be addressed by a tummy tuck or abdominoplasty. The roundness, which starts just above your belly button and extends inferiorly to your pubis suggests that the "6pack muscle" or rectus abdominis muscles have splayed apart. This normally happens to all women, when the pregnancy proceeds and the uterus rises above the pelvic brim. In my opinion, sit ups or crunches will not improve this muscle separation.
Your belly button is lost by a hood of skin, which was an expected result from the babies' growth and development. It is really difficult to reshape the belly button, without a tummy tuck. In your case, a major tummy tuck could be helpful in restoring your trunk. This involves separating the excess skin and fat from the pubic area to the base of the ribs, repairing the muscle separation, making a new opening for the belly button, possibly a small amount of liposuction around the hip area and making a fine lined closure, which would span from hip to hip.
It's a medium sized undertaking and you should discuss any health issues, smoking status and child care duties with your surgeon, so that you can get a beautiful, long-lived result. Good luck.