I just noticed that my 9-yr-old's feet roll inwards due to poor arches. Should I take her to the pediatrician first, or go straight to a podiatrist?

Podiatrist. The specialist in this matter is a podiatrist. He/she is best equipped to evaluate your child and decide if he needs orthotics- and what type.
Primary care first. You should take her to her pediatrician first. Her pcp may have recommendations to help, or a specific podiatrist she knows well and whom she trusts. Her primary care doctor will also make sure the problem gets followed up on, and will keep the records in a central location.