Yep. Smish is right.
But I can't ever resist adding my reading teacher 2 cents!
There are two ways that children learn new vocabulary:
1) Mom or Dad teaches them a word. Usually, Mom uses the word, Junior asks, What does that mean, and Mom explains it. That kind of explicit instruction in the meaning of words accounts for a small fraction of the words kids learn--maybe 10%.
2) Kid hears or reads the word many times and gradually develops an understanding of the word. This type of learning accounts for the vast majority of the words most of us know.
So obviously, the more a kid reads, the more vocabulary he/she will build. If you've heard anything about "bridging the gap" between kids who live in poverty, speak a second language, or in some other way start school at a disadvantage, and those who have educated, involved parents--well, the difference is actually only one thing: vocabulary. Kids who don't hear a rich variety of words in the first 5 years start school knowing about one-fifth as much as the advantaged kids.
Then the kids start reading. The ones who learn to read early, tend to read a lot. The ones who struggle, tend to avoid reading. So by the time kids reach fifth grade, the exposure to language is incredible. Some researchers estimate a difference of more than 100,000 words between voracious readers and the others. And with vocabulary comes understanding--of the world, of history, of connections between ideas. Reading comprehension depends on vocabulary.
Anyway...back to your question. If you are targeting younger readers or reluctant readers, then obviously you need to be very, very careful about the words you choose. Those kids are most likely to get stuck and lose their comprehension of the story if there are too many words they don't know. If you are writing something more advanced, along the lines of Septimus Heap or Inkheart, for example, then the sky's the limit. Kids who will read a 400-page book can handle the challenge. They love words and get a kick out of learning new ones. My 10-year-old and his buddies regularly talk about their "arsenal" and their "minions." (They're taking over the world...)
The other problem with trying to use only the words most kids know is that vocabulary comes with experience. Kids in California know a heck of a lot about plate tectonics. Kids in Kansas probably aren't paying as much attention to that, but know about tornados and blizzards. So what is basic and familiar to one reader may be a challenge to another.
Look at the words you've used. Is there a synonym that would be simpler but still as effective? Then maybe you do want to change it.
Does the context of the sentence and the ones surrounding it help to make the meaning clear? Can you give a few clues? Look at how JKRowling sprinkles in so many made-up words, yet gives you enough information that you don't stop reading to figure out what she's talking about.
Keep the words you love, but make sure a kid could still figure out what's happening.
