Jan Field's KidMagWriters.com is always filled with an abundance of great information. I found the following today as I was reading:
Storytellers: Character Creation
By Jan Fields
Most stories for young people have one main character, the protagonist. Longer forms, such as novels, can handle two main characters and sometime when an author is attempting to capture an old-fashioned voice (in a novel), you’ll even find the story jumps from head to head of many characters. However, it takes room to make those things work and magazines stories are short and tight – which is why you see single main characters in almost all magazine stories.
Jan's article is filled with great suggestions on creating characters depending on age level. To read the rest of this article or explore KidMagWriters.com, go to http://www.kidmagwriters.com/technique.htm