I have pretty much the exact same scar.
I was born with an intestinal blockage and the doctors applied too much pressure on my barium enema and it burst my intestines. So, the day I was born they had to open me up, take out all my little intestines and clean everything out (still, to this day when I get x-rayed below the nexk the doctors freak out if I forget to explain ahead of time).
I have exactly the same thing, scar exactly above my belly button going across my belly. Honestly... there nothing I've heard of that can be done. As she grows, it will get smaller (actually she'll just be getting bigger) and much less noticeable, but it will always be there, and it will always be much harder to get nice looking abs (believe me, I was a stage actor dancer, this was REALLY upsetting for me). You just... have to do twice as many crunches/cardio, and start making up more interesting completely unbelievable stories ("I used to way 1,500 pounds, I had a tummy tuck" - "I was attacked by a pack of wolverines in Florida. Yeah, they do exist down there."). But... life goes on. I hate that she has to go through that, it must be harder as a girl, mine doesn't pooch out a whole lot anymore unless I've been slacking on the gym, but it did when I was about her age and I was slightly pudgy, it just looked like I had fat rolls when I wasn't nearly pudgy enough to have fat rolls.
No matter how much time I spend working out or trying to look thin and fit, it still looks like fat rolls when ever I lean forward without a shirt. It's just.... yet another thing you get used to dealing with. I'm sorry for her, wish I could tell her some awesome trick for it :-(
