Dunno if you're still looking for opinions on this, OP, but...
In my Upper MG WIP I've been lightly scattering more advanced words in.
Words like "turbid water" instead of saying "dirty water".
I saw in some blogs that this was alright, but I want to hear it from you guys. My WIP is told in close 3rd if that helps any.
Thanks in advance!
Do you need the word?
By which I mean, if it's the right word, yeah, readers can look it up, be they adults or kids. Or, more likely, they'll figure it out by context. I've always found kids do this better than adults anyway.
But is it the right word? Because in your example, "turbid" is a word that isn't often going to give you much that "dirty" or "murky" wouldn't. Maybe a slight bit of tone, particularly for your narrator, if it's the sort who'd use a word like turbid in conversation. Some are.
I don't think you have a vocab problem because of it, but I would be cautious that you're not overwriting. The more specific, accessible word is usually the most impactful. So unless you have a larger need, try "dirty" and then get to the point of whatever scene. This is one of those things you might seek critique on, as I can't really say anything definitive without seeing an excerpt. Just a general caution not to overwrite just for the sake of it, whether you're writing for adults or kids.