Keller is expressing a certain reluctance in writing about her life, especially the part when she was very young, before the fever that took away her sight and hearing. This is a little unexpected, since she is a celebrated person and engaged in writing an autobiography. She says that she is afraid that significant moments of her childhood she might misremember; other significant moments she has forgotten completely. I don't think she hesitates because of a fear of inaccuracy, though. When she talks about the "veil" that is around her childhood "like a golden mist," she means her memory, but more than that, she is trying to explain the fragile and precious nature of these memories. These are very personal things for her to share, and in writing them down, there is a sense that they might become damaged or abused at the hands of her reader. Yet Keller, who for years was shut away from communicating with anyone in any form, must take that chance. So the tone of this first chapter is happy, but tinged with apprehension and a certain vulnerability. Mostly, what I get from it is a sense of Keller's amazing courage.