[Hectic]

Some peoples children.
I would absolutely contact the police, hell id show up at the local station. Ask for the staff sargent. Introduce myself, explain that i am an airsofter who has been involved in the community for 6plus years. Tell them i came across something disturbing on facebook that could jeprodize the reputation of the airsoft community as a whole and point them in the direction of the facebook page that has the...umm event i guess, posted and tell them the airsoft community as a whole in no way condones this type of activity and that after many attempts to sway their stupidity they still seem to consider doing such a foolish thing. Point them here to see that hour peers also have shown their dissagreement and let them do what they will with the info.
also if yer able. Try to attend the event (without a gun of course) and try talking to them befor they are dumb. Or just to call the cops snd news as soon as it starts going down. Then when the news shows up explain to them that you (and we as a community) do not condone or endorse such activity and made multiple attempts to put a stop to it befor it even started. Yes its alot of work but its baisically what one would do if they seen a crime happening or overheard someone planing a crime and really thats what this is people are planing to commit a crime and talking about it in a public domain for all to see someone is bound to snitch.
Good luck!