I think this is a really interesting question - because if we avoid using advanced vocabulary with children, then when are they supposed to learn it?
I think the answer is that it's a matter of quantity and proportion so the reader doesn't lose their flow or end up missing something important if they just keep reading, and also of giving the reader the chance to guess the meaning from context, so they have the opportunity to learn the word without having to look it up.
I remember reading a book when I was a child that introduced the word unanimous, though it did it in a very overt way, where the character was told the meaning of it, then used it to their great pride later in the story. There may have been places where they used it wrong to comedy effect, as well. I did not know the meaning of the word before, and I've never forgot it since.
Obviously that's a bit heavy handed for too many words, but you can often make the meaning clear through context, e.g.
"Three cowries?" she cried. "There's no way it's worth that much!"
Most people will understand that in this context 'cowries' means some form of currency.
If you can try to do this with advanced vocabulary in your writing for young people, there's a good chance you will be doing a great service to help them increase their vocabulary range.
Quantity / Proportion
However, even if you just drop in a handful of 'difficult' words throughout the novel without giving any hints, I don't think it would cause any great concern to children, who, in my experience, have a great skill in filtering out things they don't understand right now and carrying on with their day.
The problem will only come if there are so many words they don't know that they end up distanced, disengaged and bored.
In case you haven't come across them, I thought I'd mention that there are quite a few estalished readability formulas, where you can analyse your text for generally accepted readability levels for different ages.
Here is one, though if other people have links to better ones, they would be very welcome: