Planks work entire midsection!
Well it's the most difficult part.
Too be honest if you want to see your lower abs you will have to burn all the fat that hide them. Doing only workout for abs will be useless for that. You will have to make work your biggest muscles for that and it's the glutes and the legs.
The other important part for that is your diet but i think that is not a problem for you.
So do theses workout
And say good bye to the ugly fat that hide yours lower abs.
I have some great workouts in my collection you should check them out. I think they can help you.
Flutter kicks are awesome mixed with any ab exercise for lower abs