Yes, you can test for vision issues at this age! I would suggest you go to someone who is used to working with kids.
My MG gifted nephew had a very uneven profile when he was evaluated. His mental age ranged from his chronological age to 3 years above. The profile hinted to some visual issues (he had difficulty distinguishing foreground/background, he had a very hard time finidng hidden objects in drawings such as Hidden Pictures magazine).
It was discovered that he needed glasses, but he also required some therapy. They had to find a developmental ophtalmologist for that. In 6 monhts or so of therapy, he caught up. The glasses, of course, he still wears.
Maybe with your DD it is a vision issue, you can easily test her vision. However, it is very likely personality/interest, etc. It is very difficult for some of these kids to have to go through this boring easy readers when they are being read super interesting material by their parents.
