Hester and Pearl go to the forest because Hester feels obligated to warn Dimmesdale of the danger that lies in his own household--Chillingworth. After Chillingworth reveals in the previous chapter that he seeks to destroy Dimmesdale, Hester realizes that she must tell her ex-lover of his enemy's intent. She takes Pearl to the forest because she discovers that he will be walking there and also knows that they will be able to talk privately. Hawthorne chooses the forest for their reunion because it is the place of nature's truth (look for the role of sunlight in these chapters).
The ex-lovers almost argue because Dimmesdale strongly chides Hester for allowing him to live under the same roof as such a creature. Naturally, Hester does not take this well because she has had to bear the brunt of the town's shunning while Dimmesdale has made a high position for himself. They also disagree about Pearl, but after so many years, Dimmesdale does not really have a say in how she acts or is raised.