Try these tips on writing realistic dialogue. More important are rhythm and repetition — how the story sounds. Keep it simple, make it sing!
As you can see, books for younger children are much shorter. The more comprehensive overviews are things like this article on how to get a book publishedor this one on how to find an agent. Texts vary in length and complexity, depending on the age and fluency of the reader, but tend to be between and words long.
Ideas often spring up this way. Sometimes, that might mean asking for help from a grown-up, but the decision to conquer should always come from the child.