In The Poisonwood Bible why does Orleanna describe herself and her daughters from a third person point of view at the start of the book?
The third-person perspective that is given serves several possible purposes. The first is to create an eerie sense of foreboding, that someone is watching, observing them on their walk through the jungle. This sets, from the beginning of the story, a rather tense mood. It is almost as if there is someone or something up in the trees, watching them and waiting to harm them. This goes along with the theme of Africa being a person, or being a living, breathing entity that has power over people. Throughout the book Africa is not painted as merely a country, but a living force that fatally impacts people. So, the third-person enhances the mood, and the theme of Africa being alive.
Another reason it is used is for symbolism. It describes an event in the book that we don't hear about in any other place, and during that trip, Orleanna sees the rare deer in the jungle. This symbolizes Africa's untouched, primitive, wild self as it was before humans came and conquered and cut it into pieces. Seeing that scene from an outsider's perspective allows us to see the symbolic "white man" disturbing the undisturbed, unconquered Africa. Kingsolver uses it as a symbol for her major theme of colonization; doing it in third person allowed us to see every angle and perspective, and do our own concluding, free of the thoughts of any of the women involved in the scene itself. We can form our own conclusions, instead of getting someone else's.
I hope that those thoughts help to get you started; good luck!