Kids usually learn Combinatorics in high school. However, when I presented this 4th grade kid (now in College) with her favorite topic, she was able to understand the concept and apply it.
When kids make mistakes, I seldom present them with the right answer. Instead, I ask them a series of questions, as seen in this lesson snippet, until the point they realize what mistake they might have made. This reinforces their understanding of the topic, and instills confidence.
When middle school kids learn about square and cube roots, they tend to think square root is dividing a number by 2, and cube root is dividing a number by 3. Watch how this kid needs some time before he says “aaah” (now I get it!).