Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is spreading worldwide and is a public health problem. Although real-time reverse-transcription polymerase chain reaction (RT-PCR) is gold standard for diagnosing coronavirus disease (COVID-19) and there are many reports discussing it, reports about loop-mediated isothermal amplification (LAMP) tests for SARS-CoV-2, especially in children, are limited. We report the test results of three children with COVID-19 in a family cluster and assess the results of LAMP tests. The LAMP results of these children showed a sensitivity and specificity of 63.6% and 100%, respectively, that was relative to the RT-PCR results. LAMP tests using nasopharyngeal swab (NPS) and RT-PCR were almost consistent throughout hospitalization in the school children, except in the very early stage of infection. The preliminary results suggest that salivary samples would be less sensitive than NPS for LAMP testing in the late stage of infection, and that LAMP would not provide accurate results in neonates.