Truth. Stories are always about people, however we represent them. Without characters, in whatever ways we choose to draw them, there is no story.
Here's another truth. You have only one mind to work with--yours. Every emotion, every action, every snippet of dialogue is filtered through one set of experiences--yours. Why not use that consciously to create characters that resonate?
During a recent online class discussion on developing characters, I remembered this exercise:
1. Take a sheet of paper. Write down three characteristics of someone you think of as heroic.
2. Now write down their opposites. So you will have 3 strengths, heroic qualities if you will, and 3 weaknesses.
3. Now take one of those weaknesses and give them to your protagonist. Is any of those weaknesses yours? That's the one to use. Then you will have its opposite to shoot for as the emotional space into which she can grow to.
Writing fully rounded characters who also happen to be children or teenagers calls for us to channel our inner youth in a way that no other kind of writing does. We have to be close enough to childhood wants, desires, traumas, fears to give them to our young characters. But the operative word here is "give." You can't let real truth stand in the way of emotional truth. You can't hang onto the people or places who inspired your story.
And then there's another truth. The language you have at your command is not an eight-year-old's. It can't be. No 8 year old would read a story, if it were simply a transcription of reality. So you have to cultivate voices that are believable, but fictional. Experience, age, and an expanded vocabulary all give you story insights beyond that of your character. Use them as tools for building sympathy and empathy. Sometimes, reality is overrated. It's why we have fiction.