There is no magic cream but exercise. Those gimmicks they have all over the place about creams to tone up your skin does not only waste your money but also your moral if you dont't see result. I have 3 children and after 3 kids my tummy will never be as tight as before my 2nd or 1st kid. All we can do is exercise, sit up, even thou its one regime I hate the most. Your food intake is very important but since you lost most of your post natal weight that is very good news.
If exercise and diet still won't give you the prenatal tummy you want, just see it as part of being a mommy.