Alone together on the grounds of a large manor, never leaving or seeing or hearing from the world beyond cope with a small town or a city?
Are there adults on the grounds (I know you said "alone" but children don't raise themselves--not very well, anyway) who have experienced the outside world? Can the children read and do they have access to a library on the manor grounds? Either of these things would give the children some sort of insight into the outside world and they may decide that it sounds horrible or it sounds amazing, but once they get into the world I suspect there will be equal parts awe and fear. Depending on the situations you put them in, of course, one of those feelings will come to the forefront.
They had their grandmother but she was abusive, they killed her in self defense and left the estate. They had access to a libary. And are you on tvtropes your name is familar. Also how could these kid's end up in the foster system?
Then they will have some idea of how society works, depending on how outdated the books in the library were. Keep in mind that these will cause them to have some preconceived notions about what the outside world is like, so some things may be a lot more wondrous than they expected and some things may be a lot more horrible than they expected--again, this goes back to my "awe and fear" point. The foster system is going to be something you will need to study as it pertains to the area these characters are living in, I'm afraid. I've only ever known a few children in foster care but, at least as I understand it, your characters would likely be found by Child Protective Services (or the equivalent for their area) fairly easily. Someone will see children wandering alone and report it, or even confront the children about it and (not knowing any better) they may tell that person what happened. No, I am not on tvtropes--my username isn't all that unusual, however, with the current popularity of post-apocalyptic novels, movies and video games.
So do you find that armed conflict is in a state of stasis? And the kid's have magic and have never been exposed to technology.
I'm afraid I don't understand your first question--armed conflict is never in a state of stasis or it wouldn't be a conflict. The children having magic does change things a bit, but how they react to the world around them will still be a mixture of awe and fear. The advantage that they have, if you could call it one, is that they can use magic to defend themselves and keep themselves safe. I think that what you have them do is going to depend entirely on what the goal of the story is. Are they trying to find lost parents/relatives? Are they trying to get help from someone? Are they trying to get somewhere in particular? Are they just trying to make it in the world? You can make magic be as small a part or as large a part of the story as you want, so long as it fits.
Sorry I thought you'd be a fallout fan (it has the famous quote "war never changes") from your name. And how might they react to seeing other human beings for the first time?
Ah, I actually am a Fallout fan but I have been too busy to play or try Fallout 2. I think that they will understand the concept of people looking different and acting different, despite their isolation, but they won't be able to pick up on social cues. You may want to study how autistic children interact with people because that might give you a starting point to work from.
There has been since Cogito was a pup, a disparity between the have and the have nots. Your characters set you up in the enviable position to exploit fish out of culture shock. Their bucolic cuddling left them ill prepared for the Vampires of Planet Diptron, the horror they encounter a gazillion times more gruesome than the Poe their tutor Miss Crabapple had them read.
Also how could their grandmother have kept them from running away growing up? Her witchcraft is basically earth manipulation if it helps.
Questions like this are what a writer asks himself or herself. The answer YOU come up with is YOUR story.
