Families. Can't live with em, can't kill em. The only realistic tool in the bag is denying them access to their grandchildren.
I'm not Phil Mitchell, some ****ty family moral code wouldn't stop me, I'd be sending them to sleep without consideration. Although I'd probably have more respect for myself if I could keep my aggression under wraps, in this case I probably wouldn't be able to, but in my head it's my family and I can't stand most of them outside of my immediate family!
You could take 2010 Tops Dog and John Humbles Tape recorder - (just trying to lighten things up a bit) best thing to do for me would be for that family to stay well away from them grandkids included and that will hurt them more than tw**ing them.
Sounds to me like too much alcohol at the time.. Cool off for a while as a few other posters have said don't let them see the gkids. Then I can assure you the whole mess will resolve its self minus the violence.