Read what has already been written. If you want to learn how to write from a fifth grader's perspective, one of the best things you can do is read books intended for fifth graders. Examine the point of view used by each author and note important facts you think will be useful to you in the future.
Observe fifth graders. Yes, you were a fifth grader once, but chances are that was a long time ago. Times have changed. Kids have changed. If you are going to write from a fifth grade point of view, you need to familiarize yourself with how they walk, talk, and act. Take time to observe some fifth graders in various settings and keep a notepad handy to record your observations. Pay close attention to vocabulary and body language.
Tell your story from the child's perspective, not from an adult's retrospective. You, as the author, should never enter the story. At no point in time should you make your reader aware that the story is actually being told by an adult. To do this, it is imperative that you not think back to when you were a child. Instead, you must become your character. Become the fifth grader in your story. See what she sees. Hear what she hears. Know what she knows.
Do not become overchildish. It is easy to get so wrapped up in making the story age-appropriate that you flow to the other extreme. For example, fifth graders do not use the phrase "go potty." In your attempt to speak on a child's level, avoid speaking down to children or sounding condescending.
Portray a realistic childhood. As an adult, you can look back on fifth grade and realize that the problems you had then were really of no consequence. However, when writing your story from a fifth grader's perspective, you should remember that even though the problems do not seem like much to you, they are overwhelming for your character and your readers. Homework, peer pressure, and struggling to fit in --- these are real issues and should not be watered down or belittled.
Have a fifth grader check your work. She will be able to tell you a lot about your story, such as whether the plot was interesting, if the characters were realistic, and whether or not she could follow the plot and understand the vocabulary. Use the criticism to fix any problem areas and improve your work.