When it comes to getting the best training for a subspecialty like pediatric endocrinology, the most important thing is where you do your fellowship (undergraduate -> medical school -> pediatric residency -> pediatric endocrine fellowship). Pediatric endocrinologists are in short supply. The sad fact is that endocrinology (like other nonprocedural specialties) is not a very popular career choice because it requires 3 extra years of fellowship training but doesn't pay better than straight pediatrics. Fortunately, your goal is driven more by a passion for caring for children with diabetes than making money. As I write this, excellent pediatric fellowship slots remain unfilled for summer 2018 after the fall match (see https://www.pedsendo.org/education_training/residents_fellows/current_openings/index.cfm) .
So, as long as pediatric endocrinology remains underpaid, chances are you can land a good pediatric endocrinology fellowship slot even if you go to your state university and medical school (traditionally your best route for avoiding loan debt).
BTW, I'm an adult endocrinologist, but I've worked with peds endocrinologists and have volunteered for years as an endocrinologist at diabetes camp. In my private practice, I haven't taken care of young kids, but a lot of kids do cross over to adult endocrinologists during adolescence. In smaller cities, there aren't any pediatric endocrinologists, and adult endocrinologists see even the younger kids.