The best way to understand child development is to adopt a flexible, contextual approach, understanding that development happens as a child's neurobiological system interacts with many factors (parents, teachers, peers, schools, communities). Old concepts such as "nature vs. nurture" have given way to more dynamic and complex ways of seeing child development as occuring in a complex web of dynamically interacting risk and protective factors.
Recognizing the importance of inherent tempermental traits that a child brings to his or her interactions (such as emotional intensity, persistence, adaptability to change) - and the fit of these traits to that of others in the family, for example, can shed light on why some family relationships go smoothly while others are destined for frequent frustrations and misunderstandings.
Placing these dynamics in broader socio-cultural context can help to shed light on how risk factors such as poverty, high family conflict, mental health and substance abuse issues, discrimination, negative influences of the media, poor parent attachment, trauma, etc., might accumulate for some children and lead to poor outcomes. However, understanding the potential negative effects of risk factors may shed light on how to offset risks via enhancing the presence of protective factors such as improving attachment relationships, educational opportunities, and and enhancing stability and social support.
Knowing about normal child developmental milestones is key to determining if and when your child may be in need of additional supports or interventions.
The American Academy of Pediatrics' website Healthychildren.org provides an overview of what to expect across a range of areas of child functioning.