One reason it might be said that Pearl represents transcendentalism is the transcendentalist principle upholding the oneness of the universe. Pearl has an uncanny way at a young age of intuiting the link between Hestor and Reverend Dimmesdale. One might say this is due to the transcendentalist belief in an all-encompassing cosmos that unites individual parts into a single whole.
And there stood the minister, with his hand over his heart; and Hester Prynne, with the embroidered letter glimmering on her bosom; and little Pearl, herself a symbol, and the connecting link between those two.
Another reason some might say Pearl represents transcendentalism is the trascendentalist principle upholding the essential optimistic belief in the goodness of humankind and the purposefulness of life, a view in opposition with later existentialism and absurdism. Pearl has a vision of herself, her mother, and Dimmesdale united and standing before the community as one. One might say this is due to the transcendentalist belief in the redemption of goodness and the purpose of united love.
"Thou wast not bold!—thou wast not true!" answered the child. "Thou wouldst not promise to take my hand, and mother's hand, to-morrow noon-tide!"