For the two year olds it may very well be teething - molars but for the other kids just a bad habit that they never totally broke. For the two year olds can you designate a few stiffer durable toys they can chew - a chewie each or transition them to chewing on a washcloth or similar thing. For the older ones just keep with the verbal no and ideally try to catch them as they raise it to their mouth rather than after the fact. Encourage them to police each other teaching them to say "not in your mouth" if they see a friend doing it. Every kid has the right to play with clean dry toys and should have the right to protect their playspace if needed.