Why does Woolf use a line or two of monologue rather than dialogue to introduce the children in Section 1?

This technique allows the reader to really get into the mind of each character. While it is not exactly a stream-of-consciousness style, it creates the same effect, while permitting the reader the pleasure of imagining that someone is speaking aloud, though it is never clear when this is the case. It also permits the reader to infer the levels of growth in each character, by way of allowing the reader to see directly how each character thinks and analyzes the world around him or her.