Conjugate your verbs properly. The website English Tenses With Cartoons (englishtenseswithcartoons.com) clarifies this with entertaining drawings. In English, there are four kinds of present tense: present simple (I guess), present perfect (I have guessed), present continuous (I am guessing) and present perfect continuous (I have been guessing). There are also irregular verbs whose present tense changes from the standard affixes (word endings). The word "write" is one of these because it becomes "wrote" and "written" instead of "writed."
Understand the mechanics of the simple present tense. Ordinarily, if you come upon a word such as "was" in your story for a child, you might think it signified the past tense. However, words like "was" can be used in simple present tense. For example, "George Washington was the first president of the United States" is definitely past tense. However, "I am telling you, George Washington was the first president of the United States" is in the present tense because that is what the verb indicates.
Avoid breaking up the scenes in your story too much. By using the present tense, you're trying to get the reader to believe that everything in the story is happening "right now." If your story has multiple sections that take place in different times and in different places, the "present" gets moved around so much that this illusion is broken.
Decide if your present tense writing is having the desired effect by reading it once you are done. Novelist Emma Darwin acknowledges that the present tense can imbue the story with a sense of urgency, increasing the pressure on the main character. If you believe children won't get the tone you're going for, or won't think it's as satisfying you thought it would be, consider changing it.
Double-check to make sure you haven't slipped out of present tense. It can be very easy to make a mistake, so when you're proofreading, look for any verbs that end in something other than "-ed." If they're wrong, change them.