Which specialist should I see to treat rickets?

Peds endocrinologist. Pediatric endocrinologists are specialists trained to diagnose and treat rickets. Rickets is a metabolic bone disease seen exclusively in children.
Your pediatrician. Every pediatrician should feel comfortable to treat vitamin d deficient rickets (nutritional). Unusual forms of rickets, e.g., vitamin d dependent/resistant rickets, renal rickets, malabsorption-related rickets, are generally handled by "specialists.".