How do I tell the story from Dorothy's perspective?

I think you need to examine the character of Dorothy first before you try to write from her perspective. Dorothy lives across the street from Palmer and, although she is a bit younger, is the only true friend that Palmer has. Dorothy is able to keep Palmer's secret about Nipper's existence and is the one who agrees to release Nipper into safety when Beans and the others threaten the bird. Dorothy is as strong-willed as Palmer and has her own internal code of conduct that will not be reduced by the cruel antics of Beans and the others. Dorothy appreciates Palmer's love and devotion to Nipper and protects Palmer's secret despite her wounded feelings over Palmer's treatment of her when he was with Beans and the other boys. Dorothy's strength of character aligns perfectly with Palmer's own, and the two remain friends secure in their mutual admiration and trust.