Why do you think Gibson chose the particular way to end Act 2 of The Miracle Worker?William Gibson ends Act 2 with Annie singing a lullaby to...

It's an interesting way to end Act II, isn't it?  In
regards to the reasoning behind why it's done this way, I think Gibson wants us to focus
on two things:

• Annie's progression of feeling
  towards Helen


• The Keller family's progression of feeling
  towards Annie & Helen

Both sets of
progression are the same:  dislike to admiration, and even
love.

It's an interesting start to this episode as well. 
Annie is exhausted after using Percy to teach Helen late at night.  Annie has just
realized, and imparts to Helen that "all I have to teach you is--one word. 
Everything."  This is when Annie picks up the doll that causes such an issue previously,
sits down, dons her glasses, and sings a very famous lullaby called "Hush Little
Baby."

Note the progression of feeling here (especially in
regards to the quotes).  Annie begins with the doll by whispering "to it in
mock solicitude."  Moments later, Annie "lays it against her
shoulder, and begins rocking with it, ... she talks the lullaby to it, humorously at
first."  Further, Annie notes that, "the rhythm of the rocking
takes her into the tune, softly, and more
tenderly."

The progression is absolutely
amazing!  She begins by mocking and ends with complete tenderness.  This scene
corresponds exactly with Annie's feelings towards Helen.  Annie begins by mocking ("oh,
not the drawers!") and ends with Tenderness ("Jealous are you?  All right, ...") 
Point-blank, Annie is beginning to teach Helen "everything."  Often I like to go even
further about this subject and say that Helen's feelings are progressing in the same way
towards Annie:  from mocking by hiding the key to tenderly (and obediently) holding her
hand out to learn new words.

In regards to what is
happening with the family at this same moment, James, Keller, and Kate all stand
"moveless" while each of them stop and "turned his/her head, as if hearing."  Quite
simply, the family notices this change in Annie.  The family notices this change in
Helen, too.  This is another reason why the act ends this way.  Remember the disarray
when Annie first arrives!  Now look at how far Helen has come!  The family too is
approaching tenderness towards this amazing teacher,
Annie.

Let's talk about the love growing here, then.  Annie
is feeling tenderness and love for Helen.  Helen is feeling tenderness and love for
Annie.  The family is feeling tenderness and love towards Annie.  And yes, the family is
even feeling more tenderness and love for Helen, herself.