My guess is that she continues to bark at the adults for the same reason that many dogs bark at the mailman. It works, from her perspective.
She hears them. She barks. They leave. So in her mind, she scared them off. She was successful. Every time she barks and they grow quiet and leave (going into the house or the car or whatever), she is getting reinforced again, building new neural pathways, and getting better at this behavior.
The kids, however, run around and play and play and play. She can't scare them off. She probably tried that barking behavior with them a few times. But it didn't work. So it faded away before it ever became a habit.
Even when the adults aren't scared off at any particular time, if it works for her some of the time, she's going to keep doing it. It's a variable interval of reinforcement -- only the most powerful way to train a dog.
If, when you're not there, they give her any attention at all (perhaps telling her to be quiet), that's even more reinforcing. If either is afraid of her to the point that their cortisol levels rise and change their body chemistry, she may smell that. That's a change in the environment that might by itself not be reinforcing, but along with the other stuff, amps her up.
I think you're going to have a really hard time with this one, but it's doable. I'd keep her in the house when you're not home. The more she practices the behavior, the better she gets at it. We have to do whatever we can to stop the behavior from occurring. When you're home, I'd have her on a long line, call her to me the instant I see/hear the adult neighbors and reinforce heavily with favorite treats or games for come/quiet (which I would train as a single command like "hush") .
And I would train this, at first, with a 100% reinforcement schedule until she KNEW that coming to me would be more reinforcing (EVERY time) than her previous reinforcers...maybe as long as a month or two. Then I'd move a variable interval. But it would be a long time before I faded reinforcers, if ever.
Extinguishing unwanted but highly reinforcing behaviors is difficult. It requires consistency, excellent reinforcers of our own, and attention to detail.
I'm sure you can do it.