Any experienced pediatric mental health professional who specilaizes in therapy (not pharmacological treatment) would be fine. Most likely you'll be selecting eithe a clinical social worker or clinical psychologist.
Where should I start? Should I seek out a social worker or take her to her pediatrician? Over all nothing else seems wrong, She does not act depressed or angry, All three girls did well in school, and they are well behaved. Is just that the oldest seems anxious all the time.
It is not unusual for children to display some insecurity when parents are going through a divorce, particulalrly when the divorce is not accomplished in an agreeable fashion. I would not characterize this reaction as a phase, but rather as a reaction to the unpleasanr reality of divorce. It would be a help to your daughter to arrange therapy for her as a support in the face of this difficluty change in the family.