After decades of neglect, data are finally becoming available on the appropriate, safe dosing of key second-line anti-tuberculosis drugs used for treating multidrug-resistant tuberculosis (MDR-TB) in children, including levofloxacin (LVX), moxifloxacin (MFX), linezolid (LZD) and delamanid (DLM). Much needed data on some novel and repurposed drugs are still lacking, including for bedaquiline (BDQ), pretomanid (PTM) and clofazimine (CFZ). We review the status of pharmacokinetic (PK) and safety studies of key anti-tuberculosis medications in children with MDR-TB, identify priority knowledge gaps and note ongoing work to address those gaps, in the context of planning for an efficacy trial in children with MDR-TB. There is international consensus that an efficacy trial of a novel, all-oral, shortened MDR-TB treatment trial in children is both needed and feasible. Key novel and repurposed second-line anti-tuberculosis drugs include BDQ, DLM, PTM, MFX, LVX, CFZ and LZD. The rapidly emerging PK and safety data on these medications in children with MDR-TB from studies that are underway, completed or planned, will be critical in supporting such an efficacy trial. Commitment to addressing the remaining knowledge gaps, developing child-friendly formulations of key medications, improving the design of paediatric PK and safety studies, and development of international trial capacity in children with MDR-TB are important priorities.