Data show that children are less severely affected with SARS-Covid-19 than adults; however, there have been a small proportion of children who have been critically unwell. In this systematic review, we aimed to identify and describe which underlying comorbidities may be associated with severe SARS-CoV-2 disease and death. The study protocol was in keeping with Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines. A total of 1726 articles were identified of which 28 studies fulfilled the inclusion criteria. The 28 studies included 5686 participants with confirmed SARS-CoV-2 infection ranging from mild to severe disease. We focused on the 108 patients who suffered from severe/critical illness requiring ventilation, which included 17 deaths. Of the 108 children who were ventilated, the medical history was available for 48 patients. Thirty-six of the 48 patients (75%) had documented comorbidities of which 11/48 (23%) had pre-existing cardiac disease. Of the 17 patients who died, the past medical history was reported in 12 cases. Of those, 8/12 (75%) had comorbidities.Conclusion: Whilst only a small number of children suffer from COVID-19 disease compared to adults, children with comorbidities, particularly pre-existing cardiac conditions, represent a large proportion of those that became critically unwell. What is Known: • Children are less severely affected by SARS-CoV-2 than adults. • There are reports of children becoming critically unwell with SARS-CoV-2 and requiring intensive care. What is New: • The majority of children who required ventilation for SARS-CoV-2 infection had underlying comorbidities. • The commonest category of comorbidity in these patients was underlying cardiac disease.