i would suggest contacting a behaviorist. i have four dogs now, and the last one we adopted was a st bernard mix. when we got her, we had no intentions of EVER having children, so when she started becoming aggressive, we dealt with it ourselves until it became obvious that we needed a professional. -- it's very important that if your dog has, in fact, been abused in some way, that you do everything you can to correct the problem and not give her back or give her away -- the problem won't be corrected and she'll end up being put down by someone who isn't as tolerant.
behaviorists are NOT trainers. some may train also, but they should have specific experience and advertise themselves as behaviorists. veterinarians are also not specifically qualified to correct animal behavior unless it's medicine-related, and your case doesn't seem like it is. usually, behaviorists aren't that expensive and some just come to your house and stay for an hour or so. depending on your dog, one visit is all it takes, or it may take two or three. you can call your local humane society or a no-kill shelter and they should have some contacts for you.
in the meantime: i don't know how bad it is with your dog, but you could always try keeping treats with you and when a child approaches, give her a treat immediately before she even has the chance to act upset. then eventually, when a child passes, ask their parents if they mind for the child to throw your dog a treat from a distance. dogs always associate food (especially their favorite treats) with good things, and that may be all she needs.
in our case, our st bernard has made a near-complete recovery. we have a baby due in december and i'm not at all worried about her around our baby. when we got her, she had been returned three separate times and who knows what happened to her before she was rescued by the no-kill facility we adopted her from. in her case, it was more anxiety than anything, and now that she's more comfortable with her role in our family and with us as pack leaders, she's done just fine.
your problem is one that many dogs have and is usually pretty easy to overcome if you give it the effort. it will always be worth it if you do and your dog will be much happier, which is why you rescued her in the first place.