I recommend starting with cardio & stabilty exersices. The stability exersices will engage your abs without straining them. As you go start adding some strenght training & standing ab exersices. Next add weights & move to the floor. Remember when concentrating on your abs to work your back & obliques. I made the mistake of over working my abs after my 2nd child. You do not want to do that. I pulled my lower abdominals & they did not heal correctly until after my 4th child. I had lumpy abs & back issues for 5yrs because of it.
Try the workout "Drill Sergeant Mom"