What are the sources of pediatric primary care in the United States? Are these sources sufficient for providing healthcare services to the pediatric population?

What are the sources of pediatric primary care in the United States? Are these sources sufficient for providing healthcare services to the pediatric population? Why or why not? Are there certain pediatric populations that lack access to health care services? Why? What are the barriers to children in accessing health care services in the United States? Why do these barriers exist?