I'd been struggling for quite sometime with giving a nine-year-old girl character a significant flaw. She is not the protagonist, but is quite important to the story. Yet, I had trouble creating a child that was not perfect, that wouldn't be able to shed the trappings of a young Mary Sue.
Fortunately, I found a solution! I looked for conflicts between the little girl and the world's setting. The answer was in how society indoctrinates children in what is considered conventional morality, but a fictional world's 'morality' may be at odds with our own. Sometimes children feel strongly about what they think is good or evil – for example, a little kid in a religious setting might believe strongly that non-adherents of her faith are heathens and may even harbour hatred for them. This could play into the plot.
Now this nine-year-old of mine is no longer too perfect, and is a much more compelling character. Her flaw has even lent itself into a minor sub-plot now! Just thought I'd share a solution that worked for me, hope this is useful for someone else too!
Source: reddit post