You know why children's stories are harder to write than mature ones? Because children feel themes more than conceptually understand them. Kids who read young and read often are not aware of narrative techniques that are nearly impossible to explain. And the best children's stories know how to make themes somatic in a way that resonates with highly active brains. I can't tell you why kids get themes better than most young adults, but it is nearly intuitive on whether you have a grasp of themes or not.
The best way for a writer to challenge themselves on theme work is, I think, trying to conceptualize an abstract idea into something material, an object or animal. And then explain that idea through a story about the object without ever once saying how the object feels and then explore how it makes you feel and why.
