Avail yourself of the local library and read books similar to the one you are wanting to write. The best way to become familiar with books in the fourth to sixth grade range is to read some of them yourself. As you read, you may want to take notes on the author's use of vocabulary, characterization and plot.
Decide on a plot. Once you have a good idea what your age group is interested in, make sure your plot falls into that category. A plot for upper elementary is much more involved than introductory elementary novels but not as complex as a teen story. Likewise, the plot should move along rather quickly, but not so quickly that the reader loses track of what's going on. There is a fine line between bored because the plot is moving too slow and confused because it is moving too fast.
Build your characters. On the whole, it is best if your main character is the same age as the group you are targeting. In other words, if you're writing for upper elementary, your main character should be somewhere between fourth and sixth grade. Develop your characters by giving them thorough descriptions and realistic personalities.
Utilize proper vocabulary. This is where your observations come in handy. If you've done your homework, you should now have a very good idea of how your age group speaks. Use your notes and the knowledge you've obtained to create realistic dialogue and body language. Unless you are writing about a specific period, avoid slang and jargon as it can often date your writing.
Insert appropriate ideas. Kids often come up with things that adults would never think of. On the other hand, adults have the benefit of experience to educate them. In your writing, make sure that any ideas your kid characters come up with are kid ideas. Every once in a while you can use the characterization of the "brainy" child, but for the most part, it's best to leave the sophisticated ideas to the adults.
Check your work. Once you've finished your story, one of the best ways to gauge that you've done a good job keeping it age appropriate is to allow a child in your targeted age group to read it. If she reads it with no problem, chances are you've done well. Another method of checking your work is through various websites. On these sites, you can submit your work for free and they will inform you what level your story is written on.