This is a multicenter, two-cohort trial to assess the pharmacokinetic and safety profile of a single dose of ZX008 (fenfluramine hydrochloride) oral solution when added to Dravet syndrome treatment regimen containing VPA and CLB, with or without STP (Cohort 1), followed by a randomized, double-blind, placebo-controlled parallel group evaluation of the efficacy, safety, and tolerability of ZX008 as adjunctive therapy for seizures in children and young adults with Dravet syndrome (Cohort 2). Cohort 2 will not be dosed until the PK and safety data from Cohort 1 have been collected and evaluated. The PK and safety data from Cohort 1 will determine the dose of ZX008 to be used in Cohort 2.