Childermass, in between being a thief in a child-gang and meeting Norrell, engaged in sex-work. Maybe he recognized he was already inclined a certain way and decided he might as well get paid for it, maybe he thought (rightly) that getting access to gentlemen's beds would mean easier access to gentlemen's books, or maybe he just needed quick money for food. Maybe Lascelles finds out about this and attempts to hold it over his head? I just have a hankering for ex-prostitute Childermass. But please no Norrell hiring Childermass for sex.