I had two very large babies (10lbs4oz and 9lbs4oz a month premature!) and I my belly looked like a partially deflated beachball.
Proper diet CAN help you lose weight all over, so you will lose some inches around your middle if you lose a decent amount of weight-as you will lose inches all over the body.
Now here is the bad news-you have to strength train your body to firm it up-especially the abdominal region after childbirth. If you want it to be firmer and flatter-not just smaller-you simply have to.
Whether you choose a traditional strength training routine using weights for the entire body, or you choose a method like Pilates-that is totally up to you. Both can effectively help the abdominal region.
I do find that if you have had multiple children that you must do a routine that also targets the lower abdominals (below the naval) because the skin and muscles that have stretched and weakened so much there. If you would like some suggestions on workouts for the abdominals-just pm me, I'd be glad to suggest some based on what you need as far as time, fitness level, etc.
Even women who have had c-sections can flatten their abdominals and improve the area around their scar. (My dance instructor and my mother have both had a c-section!)
My abs are flat now and have great definition after a couple years of work. I DO still have a little loose skin below the naval from it being stretched during pregnancy, but I just keep it moisturized to make the texture look a little better (with a firming cream/lotion you can get at any Wal-Mart) and it helps the appearance a little.
I used to hate the skin-but being a belly dancer, I actually like the extra "shake" it lends to my shimmies now-because I don't have to work so hard when I do them.
I find that when my abs are firm and strong, and I keep the skin taken care of with lotions-that it really doesn't look bad at all.
Good luck to you-and if I can help you in any way-give me a yell.