Is it best to become a pediatric nurse before a pediatrician?

It depends on what your goals are. To become a pediatric nurse you'll have to get a Bachelor's degree in Nursing and get a job.

To become a pediatrician you'll have to get a Bachelor's degree, sit for the MCAT and apply for medical school. After medical school you apply for your specialty.

If you wanted you could work as a nurse to pay the way for medical school, however you'll have to double your education time since you can't work full-time and go to medical school full-time. Also nursing doesn't translate over into the medical field, we use a different set of diagnoses and we focus on different aspects of patient care.