In Chapter 17 of Jeanne DuPrau's The City of Ember, Doon, Lina, and Poppy make it safely in boats through the wild river ride to a walled off pool of water. They then pull to the edge of the pool where there is a rocky slope, and Lina, wading ashore to explore, finds a path to follow. But before they follow the path, they stop to eat. It is at this point that Doon says to Lina, "I saw the guards run after you. Tell me what happened. ... And what about Poppy? What did you tell Mrs. Murdo?"
Lina then explains how she was caught by and escaped from the guards and that, while waiting for the lights to come back on, she made the decision not to leave without Poppy. She further explains she "told [Mrs. Murdo] the truth," or what Lina hopes is the truth, which is that Lina is taking Poppy to safety. Before the lights went out for the longest time, Lina had thought leaving Poppy with Mrs. Murdo would be the safest thing to do, at least for a time. But, while standing on the roof of the Gathering Hall, waiting for what seemed like an eternity for the lights to come back on, Lina had realized the following:
There is no safety in Ember. Not for long. Nor for anyone. I couldn't leave her behind. Whatever happens to us now, it's better than what's going to happen there. (Ch. 17)
In other words, though Lina had thought differently at first, Lina had realized Ember was too much in peril to leave Poppy safely behind. Even though making the journey was more difficult with Poppy, she knew taking Poppy along was the right thing to do.