Although rapid genomic sequencing (RGS) is improving care for critically ill children with rare disease, it also raises important ethical questions that need to be explored as its use becomes more widespread. Two such questions relate to the degree of consent that should be required for RGS to proceed and whether it might ever be appropriate to override parents' decisions not to allow RGS to be performed in their critically ill child. To explore these questions, we first examine the legal frameworks on securing consent for genomic sequencing and how they apply to the specific context of RGS for critically ill children. We then use a tool from clinical ethics, the Zone of Parental Discretion, to explore two case studies and identify under which circumstances it might be appropriate for parental refusal of RGS to be overridden. We argue that RGS may be a context where, in addition to assessing the complexity of the test offered, it is ethically appropriate to consider an effect on patient outcomes when deciding the degree of consent required. We also suggest that there are some contexts where it may be ethically justified to perform RGS, even when it is actively against the wishes of the parents. More work is needed to examine exactly how 'time-sensitive' exceptions to current guidance on consent for genomic sequencing could be formulated and operationalised for RGS for critically ill-children.