A developmental psychologist studies how people change across the whole lifespan: how thinking, language, emotion, attachment and social understanding develop from infancy onward, and what happens when that development follows an unusual path. Much of the discipline sits behind the scenes, in research that shapes the assessments and interventions other professionals go on to use, rather than in direct contact with clients.
Research feeding practice
The tools an educational psychologist uses to assess a child's learning, or a clinical psychologist uses to understand attachment difficulties, are built on developmental psychology research, even though the developmental psychologist themselves is rarely the person in the room delivering the assessment. Applied developmental psychologists do work more directly in education, early years services and research settings, but this remains closer to service design and research than to individual treatment.
If you are looking for a child's assessment
For anyone looking for an assessment or treatment for a specific child, an educational, clinical or child psychologist is the more direct route, since these are the practitioner roles built around individual client work. Developmental psychology is worth knowing about as a discipline, but it sits one step removed from the professionals most families will actually meet.