So... if I was the spymaster for this country, I would employ the bartender to listen in on all the information brokers conversations. Sure you might not pick up everything, but something like this drops in your lap, and just paid for the bartenders salary for the year...
Irregardless of the bartender being a spy or not, the dude is still standing there behind the bar as you threaten on of his more frequent patrons... not to mention anyone else within earshot.
On a side note, while I normally dont condone the murder route, people who can look the other while when children are used as hostages and sacrifices are no longer really humans in my eyes. Some people are just too far gone to let live. If what happened at the fort wasnt warning enough, maybe there should be a change in the leadership... after all the king, his nobles and the priests are much closer and likely a whole lot weaker then the supposed "three demons"