I was tormented a lot as a kid...very few punches, mostly name-calling. Finally, during the early part of my sophomore year of high school, I finally realized what it would take to stop it. In general, I just ignored the b.s. But a few well placed borderline-insane remarks, intimidation tactics, and a massive shift in confidence that would be obvious to anyone seemed to stop the problems dead in their tracks. I have had few problems since.
Ep, I want to delve a bit more into all this. Grandview is a relatively middle-class suburb, right? Low crime? White collar or working class? And what about your kids? Well-liked? Nerdy? Fat?
Based on the sucker punch, that should be enough for an assault charge on the kid. Did you file a police report and charges? At the least, he'd have to spend a few nights in Juvi. And you're in Jackson County, right? If he's a little middle-class snotrag, that might scare the shit out of him.
Do you have a lawyer? Have a lawyer send a certified letter to the kids' parents/guardians insisting that they cease and desist. Equip your oldest with an emergency cell phone so that they can call 5-0 at the first sign of trouble.
And there's always strength in numbers. And I'm not talking about getting 5 big guys to go fuck those punks' worlds up. I'm talking about just having several friends around them when they're out, if possible.
I'll write more based on your responses to this.
(In Sycamore's Utopia, he would put bullets in the back of the kids' heads, then chop their bodies up and toss the pieces into the Missouri River. The parents, guardians, extended family, and even pets would not be safe. But this is reality, so I hope some of my legal suggestions help.

)