Make a list of the traits you want your child character to have. This list should include physical characteristics, such as stature, hair color, facial structure and physical expressions of the characters, as well as the style, hobbies and thoughts the character may entertain or exhibit.
Make a simple sketch of the character by drawing the outline of its physical figure. Pay attention to the proportions, since equality in physical features is one of the most important aspect when drawing humans. Since you are creating a character, it is alright to emphasis certain aspects, similar to how you would if you were drawing a caricature. If the character is an athletic child, you may want to emphasis the size of his muscles or if it is a smart character, you may want to make the size of his head larger than the rest of his body.
Add in details to the sketch that will give the child character personality. Draw clothes on the character and include accessories according to the character's hobbies. Make sure to include child-like features, such as an ice cream cone dripping from her hand or a bow in her hair. Childhood characters can sometimes act like adults in their adventurous stories, but you must distinguish them as children in these smaller details.