Hi Nightmare (I have to ask: why "nightmare"?..is English a nightmare??)
"order" is one of the subjunctive verbs (as you seem to know) and in the noun clause that follows a subjunctive verb ("that the slums be demolished") the verb is given in the simple or "base" form; in this case: "be".
Most grammar texts and I'm sure a few websites (maybe this one?) give lists of "subjunctive verbs".
Student or Learner