After the children of a disgraced and imprisoned lord escape confinement and are traveling to find a relative to take them in, the older of the two, a sister, is thinking of how they are just kids, vulnerable to just about any threat out there. Just kids. That kind of threw me. Would a 12th century child of royalty think of herself as "a kid?"
The answer is: no. The word kid as an informal designation for a child didn't come about until at least the 1590's. And there were other, more modern words and phrases peppered throughout the novel that frankly, took me a little out of the story, which, as a writer, is a no-no. This led to an inner debate about voice vs. language.
See, voice refers to an individual writer's style. Having read most of Stephen King's work, I know his voice. No matter what kind of story he's writing - Western, Sci-fi, Detective, Horror - take his name off the cover, and I'd know it's him. And most people have favorite authors like that. It's a sense of humor, or gravitas, or clever turns of phrase, or poetic sensibilities. Each writer's voice is as distinct as one snowflake is from another. It's one of the things that makes them a favorite.
An author's voice is different than the language and dialogue of the work itself, especially in a period piece. A knight in the middle ages is going to speak differently than a Navy Seal. If he doesn't, then the story lacks (in my opinion) verisimilitude. Again, your job as a writer is to make the reader forget that he is reading at all. To become so engaged with the story, that she is living the story through her senses. Nothing will bring all that to a screeching halt faster than a Knight of the Realm saying "'Sup, Dawg," to a Lord or Lady.
All I'm saying is this: pay attention to language, when it matters. Your voice will always be your voice. You can't change that, and you shouldn't. But if you're writing in some dialect, or in a different culture, or even a different time, do your research, get it right. There are people who will know the difference, and they will call you on the carpet if you don't do it justice.
Until next time!