I disagree. Child rapists are not provided protective custody. They are put among the general population. It would take an extraordinary leap in my opinion to say that the general population was 'unusual punishment'.
They are put in Genpop if there is no reason to believe that Genpop knows what they've done. This case is absolutely different, the prison's genpop cannot be used as an ad hoc death penalty, which is what would happen if this guy got what he deserved. He won't though, he'll wind up in protective custody, like a cop.