The full title of Johanna Spyri's novel is Heidi, girl of the Alps, and Heidi's full name is Adelheid. The story is rather "feel-good" and Pollyanna-like, as nearly everything bad in the story has a happy ending.
Heidi is orphaned at a very young age. Here are Aunt Dete and Barbel talking about the future for this little girl, fearful about sending Heidi to live with Heidi's rather cranky and eccentric grandfather on the top of the mountain.
"Oh, as to the looking after," remarked Dete, "the boy need not put himself out about that; she is not by any means stupid for her five years, and knows how to use her eyes. She notices all that is going on, as I have often had occasion to remark, and this will stand her in good stead some day, for the old man has nothing beyond his two goats and his hut."
It is clear from this passage, which can be found early in the book, that Heidi is only five years old when the story begins. That is part of the reason, of course, that the adult women are so concerned about leaving the girl alone with her grandfather; however, their personal wants and desires keep them from being too afraid, so they send her up the mountain where Heidi begins her idyllic Alpen adventure.