There is more than one doll in The Miracle Worker. The dolls in the play symbolize the emotions that Helen Keller and Anne Sullivan find difficult to express in other ways. The dolls often function particularly to symbolize Helen herself.
In act 1, Aunt Ev puts together a doll for Helen, but it is quickly and carelessly made and without facial features. Because Helen identifies with the doll, she is distressed that it does not have eyes and insists that two buttons be sewn on it for eyes. This symbolizes that although she is blind, Helen still has eyes, representing inner sight or insight.
When Anne Sullivan comes to live with and teach Helen, she brings her young pupil a doll. The doll becomes a proxy or symbol for the emotions each of them find it difficult to express in other ways. Helen initially throws the doll to the ground or uses it as a weapon against Anne, knocking her tooth out with it. Through the doll, Helen expresses the frustration, anger, hostility, and confusion she is feeling.
Anne uses the doll to express her emotions about Helen. When she holds the doll tenderly and sings "Hush Little Baby" to it at the end of act 2, this expresses her growing love and tenderness towards Helen, a love that foreshadows the connection the two will forge in act 3.