Background and Objective: Vulnerable populations, such as adult immigrants, are proportionally over-represented in reported COVID-19 cases and underrepresented in Covid-19 testing rates. Worldwide, evidence is lacking on immigrant children’s access to COVID-19 diagnostic tests. Immigrant children (i.e., those born outside or with at least one parent born outside the EU) are subject to adverse social determinants of health. As a result, they tend to use less primary care and more emergency services. This study aimed to analyse immigration as a determinant of COVID-19 testing among children living in Amadora, Lisbon Metropolitan Area, Portugal. Methods: We link data from a prospective cohort study, conducted since 2019, on 420 native and immigrant children born in 2015 and living in Amadora, with administrative health records on COVID-19 tests. Testing data between March 2020 and October 2022 were used to estimate prevalence ratios for having ever done a test (yes/no), through Robust Poisson regression; to estimate determinants of the number of tests a Poisson regression model was used. Results: The sample was balanced between immigrant (52%) and native (48%) children. Overall, 356 (85%) children were tested for COVID-19 during the period considered, among the non-tested group 84% were immigrants. The median number of tests undertaken was 3 tests per child (3 for natives and 2 for immigrants). Models adjusted for children’s sex and birthweight, caregiver’s sex, age, education level, employment status, and monthly household income, consistently showed higher prevalence ratios of ever being tested for COVID-19 in natives. Analogous results were obtained for the number of COVID-19 tests. Conclusions: This study, the first in Portugal to provide evidence on children’s access to Covid-19 diagnostic tests, suggested immigrant children face barriers to this service. Interventions in the large-scale forces that impact health are needed to reduce health inequalities in children.