SOLVED. Children's Fantasy: A family is running away from a cloud that is trying to take their youngest son away. The world is flat and they travel to the edge of it as they try to get away. Spoilers. [s]

[Alessa]

The story takes place in a world that is flat and where mythical creatures such as mermaids exist. This world is much more dangerous than Earth because of that. The main character is aware of the existence of Earth and is envious of the people who live there. The main character is a girl and the middle child in her family. She has an older sister and a younger brother. One day, the family is forced to flee because there is this cloud, another kind of mythical creature, that is coming to take the son away (pretty sure he will just die if the cloud takes him). The majority of the book is this long road trip as the family attempts to find a way to save their son, and they go to the edge of the world at some point, where they are disappointed to find these huge ice walls. For some reason that I can't remember, I'm pretty sure this means that they can't save their son. However, there is this emotional reveal at the end, where the family realizes that the older sister (I JUST REMEMBERED HER NAME IS MILLIE) is the one whom the cloud actually wants. After that, Millie basically just disappears. I promise this is an actual book I read, not just a dream I recall. I read this in 2017/2018, and it is a children's fantasy book. One more thing I remember - Millie is kind of moody toward everyone throughout the book until the end.

[Ann]

Are you sure that her name is Millie? Because otherwise it sounds a lot like My Diary from the Edge of the World.

Here's the GoodReads description:
Spirited, restless Gracie Lockwood has lived in Cliffden, Maine, her whole life. She’s a typical girl in an atypical world: one where sasquatches helped to win the Civil War, where dragons glide over Route 1 on their way south for the winter (sometimes burning down a T.J. Maxx or an Applebee’s along the way), where giants hide in caves near LA and mermaids hunt along the beaches, and where Dark Clouds come for people when they die.

To Gracie it’s all pretty ho-hum…until a Cloud comes looking for her little brother Sam, turning her small-town life upside down. Determined to protect Sam against all odds, her parents pack the family into a used Winnebago and set out on an epic search for a safe place that most people say doesn’t exist: The Extraordinary World. It’s rumored to lie at the ends of the earth, and no one has ever made it there and lived to tell the tale. To reach it, the Lockwoods will have to learn to believe in each other—and to trust that the world holds more possibilities than they’ve ever imagined.

[Alessa]

Thank you!