The prehospital care is complex, which means to sometimes work in unusual and unique situations. Care of children in the prehospital care is rare, why the ambulance staff have difficulty getting routine in it. Previous studies have shown that training in the area is perceived as inadequate. Aim: The aim of this study was to describe how nurses in ambulance care estimate their competence within prehospital care of children and test reliability and validity of the self-assessment instrument “Self-assessment of one's own professional skills in nurses within prehospital care in the care of acutely ill children” [Självskattning av den egna yrkeskompetensen hos sjuksköterskor inom ambulanssjukvård vid omhändertagande av akut sjuka barn]. Method: Data were collected in the form of paper questionnaires. The sample consisted of nurses who are actively working in the ambulance care. In total, 127 respondents participated in the study, those from the northern, central and southern parts of Sweden. The data were analyzed by factor analysis, principal component analysis with rotation method and Cronbach's alpha. Descriptive statistical calculations have also been carried out. Results: The nurses assessed their competence in medicine to high and in nursing to very high. The factor analysis performed on the instrument found two different factors. The charge factor showed that the instrument's validity was good. Reliability was tested by Cronbach's alpha with a value of 0.933. Conclusion: The respondents in the study estimate that they have high competence in both medicine and nursing. The self-assessment questionnaire shows high validity and reliability. For continued use one question has to be revised.