we've had the filly for about 2 weeks and have spent hours and hours in the pasture with her and the other horses -- yesterday was the first time she did this
My wife does most of the feeding since I normally have to be at work early early and it is an hour away --- I will make sure she reads this so she knows to scare the crap out of the filly in order to break the behavior
I let the my wife and daughter know that if they want to go out to the pasture - they need to bring the carriage whip with them -- since it makes the loud whistling and popping sounds if you do it right.
Will do so research on some ground work I can do with her --- I have been clipping a lead to her halter and walking her up and down the pasture
What are you in the pasture doing for hours and hours? Horses are not dogs, and doing this does nothing but cause problems. And humans are not horses either.
If you are babying the horses and this filly, you need to quit. That means petting and messing around with her too, as that makes this worse.
You also need to realize that if your wife/kid can't make it a defining moment in their handling of this filly? That means put a stop to it. They will actually be making her worse each time she does something and they allow it.
All horses should be kept away from the humans when going into pasture. Even if this filly stays back, but only the others come close, filly could well decide to rush another horse and shove that horse into human. Or wheel and kick causing a kicking melee.
You need to reassess how you are feeding, and take steps to alleviate risking yourselves until you get this sorted out.
But the main thing elsewhere is to not baby this filly at all.
And quit riding in the same pasture with the other horses is what I would do. Good way to end up in serious trouble.