Write a nonfiction article. Although it may be an appealing idea for many writers to write children's fiction, you'll find that there tends to be a lot more juvenile interest in nonfiction writing, such as articles on science, sports and nature. When writing for a children's magazine, editors like to go for nonfiction articles written in the form of a short story. Articles filled with vivid pictures and colorful imagery are always a good idea to get the editor's attention.
Keep it modern. It is also important to try to put any notion of your own childhood to the side, as your own memories will serve little purpose in relating to the children of today. Interests and hobbies might be entirely different now than they were when you were a child. For example, today's kids might be more computer-literate than previous generations.
Knowing the market is crucial. If you want to write for children's magazines, it is important that you also read what is selling. It is necessary that you get to know a wide range of popular magazines, and really build up an idea of what children actually want to read. Don't just rely on what the editor tells you, get to know your audience.