Except for the tattling part, I've been in this same situation. Kid A lives across the street from us, she's a little older than Kid B (my DD), and her parents have taught her that the best way to get what she wants is to be relentlessly annoying about it. They're the kind of parents you can annoy up to a point, at which time they'll finally give in just to shut the kid up. So Kid A and her older brother both are completely inflexible kids, and will bully, cajole, and plead whenever they're even slightly rebuffed on something they want. What they'll never, ever do is stop.
Well, I can't say never, because when Kid A was over at our house, and she tried that with DW or myself, she learned to take no for an answer from us.
Unfortunately, she never learned to take it from Kid B, and so it's been a constant circus between the two. For the most part DW and I stand apart from it and let them work it out between themselves. We coach them on how to find common ground, and we also coach DD on how to deal with conflict. When their relationship started, Kid B (then 2) would just do whatever Kid A said, because she was trying hard to be A Good Girl and she thought arguing with her friend was bad behavior. So she'd let herself get pushed around until she snapped, and there was a punching incident, and another where I caught Kid B trying to shove Kid A off the top of her slide (a 3ft drop, but still). They were enrolled in the same dance class, and DW watched as Kid A hounded Kid B over the pink star (a mark on the floor) for the entire class, while the teacher and Mom A did nothing. There was another Kid C in that class, and Mom C pulled her kid out of Kid A's dance class AND private school because of Kid A's incessant bullying. I think that may also be why she moved out of the neighborhood.
So... that's where it started, and though Kid B has learned more constructive ways to be assertive, the issues have persisted, with crying jags on both sides. They periodically hate each other, and then they miss each other and want to play again, so too much time together seems to be a problem, and brief separations seem to be very helpful. We chalk it up to kids being kids and think it's a good learning opportunity for both of them.
Unfortunately, Mom A didn't feel that way. She called up DW because Kid A had been crying too much, and decided to use it as an opportunity to instruct DW on how to be a proper mother. Her little dearie wasn't the problem, ours was, and we need to immediately intervene and solve the kids' problems like she does, blah, blah blah.
Strike one.
Family D moved into the neighborhood recently, with a Kid D our kids' age, and Kid D had been playing a lot with Kid A and B. In fact, Kids B and D are classmates. Mom A decided to go talk to Mom D and set her straight on a few things... what a terrible kid Kid B is, along with a few choice comments about Mom B, and Family B in general.
Strike two.
Mom D reported all of this to Mom B, and as it included information that Mom D could not have learned from any other source but Mom A, plus if Mom D has a fault it's being overly honest and forthcoming (I really didn't need to know how much milk she produced, thanks), the information was deemed 100% credible. So when Mom A came to apologize to Mom B and begin smoothing things over, and Mom B confronted Mom A with the things she'd said to Mom D, Mom A denied saying them, placing the blame on Mom D for trying to stir up trouble.
Strike three.
The kiddos have made up and desperately want to play together, but since the parents can no longer be respectful to each other, that is no longer an option.
Sorry, I wish this story had a happier ending.
