For each main character in your children's book, you need to create a character bible. Your character bible really lays out who your character is in terms of personality, looks, history, family, quirks — all the elements that contribute to making a person who he or she really is. A character bible functions to help you get to know who your character is and to help you differentiate between characters.
A great addition to a character bible is a speech section where you identify how your character speaks. Consider the following characteristics of speech and lay out where each of your characters falls where these attributes are concerned:
How articulate is she? (this only refers to characters who are old enough to be articulate here.) Is she educated, and is it reflected in her speech?
Is she not a native speaker? What is her native tongue? Does her English sound like she got it from a book? Or is your character a younger child apt to make grammatical mistakes such as mistaking one word for another?
What is the quality of her voice? Is it hoarse? Loud? Soft? Squeaky? High? Low?
Does she have any verbal anomalies such as lisping or stuttering?
Is she direct in her speech or shyer, more obtuse?
Does she use swear words or slang, jargon, or street talk?
Is she loquacious or terse? Abrupt or apt to talk your head off?
Does she answer every question with another question?
Though not all of the preceding questions can (or should) be answered where every character is concerned, they can be very helpful in finding your character’s particular voice and keeping that voice consistent and differentiated from the other characters in your book. Plus it helps you avoid the problem of having your teenage protagonist sound the same as his mother.