Background Most healthcare contacts for children in the UK occur in general practice. Diagnostic tests can be beneficial in narrowing differential diagnoses, however, there is substantial variation in the use of tests for children in general practice. Unwarranted variation in testing can lead to variation in quality of care and exacerbate health inequities. No prior study has tried to understand why variation in testing exists for children in general practice. Aim To explore GP perspectives on using diagnostic tests for children in primary care and the underlying drivers of variation. Design and setting Semi-structured interviews were conducted with GPs and trainee GPs in England. Methods We conducted interviews with 18 GPs and 2 trainees between April and June 2023. The interviews were transcribed and analysed thematically. Results GPs reflected that their approach to testing in children differed from adults; their threshold to test was higher, but the threshold to refer to specialists was lower. GPs’ perceptions of test utility varied, including objective testing for asthma. Perceived drivers of variation in testing included: 1) intrinsic (clinician) factors relating to their risk tolerance and experience, and 2) extrinsic factors, including disease prevalence, parental concern and expectations of healthcare, workforce changes leading to fragmentation in care, time constraints and differences in guidelines. Conclusions The findings of this study identify actionable issues for clinicians, researchers, and policymakers to address gaps in education, evidence, and guidance, reduce unwarranted differences in test use and improve the quality of health care delivered to children in general practice.