In addition to the main lost girls, there could be side characters such as wise old men or women who offer advice to the rescuers or the lost girls themselves. There might also be animals in the story. If it's set in a forest, for example, friendly animals could help the girls, or dangerous animals could add to the challenges they face.
It could be about girls who are lost, perhaps physically lost in a place like a forest or a big city. Maybe they are lost in a more metaphorical sense, like lost in life, not knowing their purpose or direction.
It could be about lost girls in various senses, perhaps physically lost in a strange place, or lost in their lives, like they've lost their way, their identities or their purpose. Without reading it, it's hard to be more specific.
I'm not sure specifically as there could be many novels with that title or a similar name. It might be about girls who are lost in some way, perhaps lost in a strange place, lost in their lives, or lost in their identities.
The 'lost girls of Willowbrook' is likely about the experiences of girls in Willowbrook. It could be about their struggles, perhaps they were mistreated, neglected or faced difficult circumstances there.