First, growling itself isn't bad. Growling gets such a bad rap. It's a form of communication, it's the only way your dog has to let you know they are either A) upset; B) afraid; C) uncomfortable with a situation or person; D) hurt or sick. So, really, when a dog growls, the growl isn't the problem, the thing you want to deal with is what caused the growl, and help your pup learn to deal with it.
So, how old is your pup? Lots of times, dogs are uncomfortable around kids. Children have higher pitched voices, they move faster and more spontaneously, and they can be pretty loud and hyper sometimes. So, if your pup hasn't been socialized or had any experience with kids, it's understandable that she might be nervous.
As for other dogs, socialization around other dogs, being where she can see other dogs, can help.
If your dog is simply barking, it can be that she just really wants to meet these kids or dogs, and is overexcited. OR, it could be that they make her nervous and scared. Either way, getting her around these things more often can help.
Now, if she is only barking, it shouldn't be too difficult. Many dogs that are excited or fearful also lunge and pull toward the triggers. What the retired trainer said is ok, but I would tweak a few things:
First, having her sit is fine. Stroking her and feeding her treats is fine, if she is calm. But, timing is key. If you stroke her when she is upset or starts to bark, she may see it as a reward for her behavior. So, don't use the stroking as a way to calm her down, wait to do it til she is calm.
If she "fails" and barks or growls, back up with her. Dogs have a threshold. In general, it's the distance away from something where they feel safe, and can be calm. If your dog "fails", in my opinion, she is too close to the trigger to be calm, so back up. Give her a bit more space away from the kids or dogs. Then, try again. The goal should be to keep her under her threshold hold, that way, everything is positive, and she is in a position to learn. If she is past her threshold and gets upset, her mind isn't really open to learning, and she may not even respond to you.
It's a good idea to head to some parking lots where other dogs may be (like a pet store lot). Stay far enough away so she is able to just see all the other dogs come and go. Same with kids. Head to a parking lot where she can see lots of kids, but not have to be too close.
And, if you know someone with children, you can get them to help. You can have them walk by the dog (she's leashed, of course), but maybe have them be 10 feet away or so. Then have them toss treats in her direction. That way, she may come to associate kids with good things, and she doesn't have to get too close to get the treat!