My question is, how would the kid be treated and, more importantly, what sort of place would they be likely to keep him? They're a rival sept of, for the time, fairly good means.
The politics in question are mainly jostling and minor power struggles between a few of the extensive smaller septs of Clan Campbell. There isn't a really nasty feud here -- just tension and some mutual dislike.
The boy's going to be rescued, and his rescuers are rather supernatural people, so there's a lot of flexibility as to what will work storywise for a place of imprisonment. Unfortunately, the kid doesn't know about the unnaturalness of his potential rescuers, so it would be good if they could get a message to him beforehand. (They've got a friend in the enemy camp who might be prevailed upon to deliver a message, but he's still got to be able to get to the kid to do it.)
Said supernatural people are quite like the fairies of Highland legend; they're a bit mischievous, and can become invisible, fly, and heal. (They aren't fairies, but they're perfectly happy for people to think they are.) They'll be the ones getting the kid out, and also having a few words with the perpetrators to let them know that they'd better cease all that could be remotely considered hostilities. Or else.
I've tried Googling various combinations of the words "Scotland Highlands kidnapping clans," sometimes including "eighteenth century," always including "-Stevenson" (otherwise all you get is pages about Kidnapped), and read every book I could find on the history of the area for general research, but this is the sort of little thing that just doesn't usually come up.
Any help will be deeply appreciated. Thank you!