In this particular type of operation, Nneoma, they are lured. Often a younger gang member, just a few years older than the girl, will chat her up as she comes out of school. They drive tarted-up cars, give the girls presents ranging from happy meals to ipads, treat them like they're special and tell them they're sexy. They share alcohol & drugs with them. More often than not, other girls their age will actually be jealous of them.
There's also a massive problem with the move towards children's homes that are just 3-bed terraced houses, run by outsourced services like G4S. Children staying in such homes are often home educated there, and the full-time staff comprises 6 adults for just one or two children. There's no-one to report the kids missing if the staff don't, and the staff often don't even if they know where the child is and what's happening to them. Some children have said they saw the staff entertaining gang members with drinks.
Some other children have overstressed parents who aren't coping. Some have conscientious parents who believe their kids are at after-school clubs or shopping with friends.
It's not too hard :(