Firstly, an Orthodontist IS a dentist!
The main difference is that they have done another University Degree. This gives them much greater skills in identifying and treating developmental disorders of the teeth.
Children get a referral to an Orthodontists specifically to look for developmental disorders of the teeth and jaws. Therefore, and Orthodontist works alongside your dentist, but will focus much more on the transition from baby teeth to adult teeth. Your dentist will generally spend their time with you, checking for tooth decay and gum disease, and helping you keep your teeth as healthy as possible.
If a dentist is worried about your kids’ dental development, they will send you to an Orthodontist, almost like a “hired gun” to check specifically for these kinds of problems. That said, there is no need for a referral to see one.