We Met In The Tunnels

Four simple rules to follow: Eight p.m. curfew, Only one child per married couple, Only man and woman relationships, and every child has to be independent by the age of 20.
Not too hard to follow, right?
Not for Sayna, a second child that has lived in the underground mining tunnels for most of her life. She has been cared for by allies of the second children for her whole life.
One day, she meets a mysterious boy, Axton, who is looking for a way out. They go on an adventure that leads them to somewhere they could never have imagined, and realized that the underground world is a lot more complex that just the lost second children of their society.