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.
The lost girls of Willowbrook were likely the female residents or patients in Willowbrook. They might have been those who got lost in the system, either in a care facility or a community setting.