In The Scarlet Letter Pearl is, indeed, made to represent several things for Hester and Dimmesdale.
The first thing that she represents is their nature. Dimmesdale and Hester engaged in their love affair because they both obviously share an impulsive and passionate nature. It is highly possible that, had Pearl never been born, they would have continued their affair in uttermost secret. This is the reason why Dimmesdale cannot connect with Pearl prior to his admission of guilt; because Pearl reminds Dimmesdale over and over that he is not the saintly man that the rest of the congregation takes him for. Similarly, Pearl consistently reminds Hester that her nature as a woman ahead of her time is what has rendered her so feeble and succeptible to the unfair treatment of the people.
Pearl is represented as a symbol in the manner in which her demeanor changes in the presence of Hester and Dimmesdale. Had Pearl bore no significance as is, she could have just acted obliviously when close to any of her parents. However, this is not the case. In the forest, she is weary, despondent and uneasy in the presence of Dimmesdale as if he were a bad omen for her, and vice versa. With Hester, she is cruel and manipulative, prompting her mother to wonder whether she had really been sent from God, or from some other supernatural force. In all, Pearl is reflective of her parents' guilt, of their fears and of their never-ending feeling of punishment.
Keep in mind, however, that one cannot just write Pearl off the narrative because it is precisely the fact that she IS a REAL human character that has caused the misery in the lives of Dimmesdale and Hester. Again, had Pearl never been born, neither Hester nor Dimmesdale would have had to bother learning any new life lessons, nor paying for any mistakes.