It's difficult to give you a thesis statement without first knowing what it is you want to express about the symbolism in this story. Here's the advice the eNotes Essay Lab gives:
The thesis statement is where you will let your readers know what position you will take on your topic. When you write your thesis, don’t be shy: make a bold and factual statement that expresses your position.
So first you need to decide what your position is. What particular point(s) do you want to make? This story is full of symbols: the stair guard that is supposed to keep children from falling downstairs actually isolates the woman from the rest of the house; the fact that she is kept in what used to be the nursery; the strange woman she sees...
(The entire section contains 2 answers and 396 words.)