What’s your character’s name? Names are important. You want a name rings true to your character, but doesn’t draw attention from the story. When writing about children, be aware of name styles. If your character is five years old, some names are no longer being used.
What’s your character look like? Size, age, hair, health issues etc. are important to give the reader a mental image.
How does your character speak? Slowly, bass/treble, formal, uneducated etc. Accent is important, but just give the readers hints about the dialect or slang.
How does your character behave? Emotional, mental, anger states are important, if only to give the character reasons for their behavior.
How about clothes? Some writers ignore clothes or lack of clothes, but they can give the reader an idea of what the character looks like.
Gives us the specifics. Giving your character specific interests/likes/dislikes personalizes them, makes then unique.
You gotta eat. What/where/when does your character like to or have to eat. Food can be regional or it can reflect a child’s whims.
What’s your character’s place? Where your character lives or visits can be important to the story line.
What’s your character’s past? Even if you don’t include the backstory in the manuscript, it can be important to the plot.
Put it together. Once you have built your character, write his/her story.