My knowledge of child development is driven largely by raising my two teenage boys, being a Boy Scout leader, and working as a substitute teacher. What this means is that I have a basic understanding of some concepts but probably cannot articulate them in a technical way.
The most challenging aspect of child development is that the rate of development differs from child to child. My two boys, who are now 17 and 15, have shown wildly different rates of development. When working with Boy Scouts, you see a broad range of development stages as you are dealing with children from ages 10 to 18. The complexity is increased because development in different areas, such as physical, emotional, and cognitive, happen at different rates within each individual. For a teacher, this means you need to have the ability to recognize students are at different stages and adjust for that difference.