In my experience, most women should start staring with a tummy tuck to get a flat abdomen.  Liposuction of the abdomen is an option, but it will not tighten those muscles that have been stretched out from pregnancy and may leave hanging skin in the lower abdomen that ultimately will require surgery anyway.   Many women who I have taken care of find that their butt looks much bigger and better with a flat abdomen and often do not proceed past getting a tummy tuck.
When you are looking for abdominoplasty surgeons make sure you chose one who is a member of the American Society of Aesthetic Surgeons.  This will give you the best chance of getting a qualified surgeon.  In addition, look for someone who uses a NO DRAIN tummy tuck technique and a 24 hour rapid recovery protocol.