This study focuses on the evolution of radon concentrations in tap water samples of primary schools in Najaf city using RAD-7 detector. Annual effective doses in children were calculated. Geographic Information System technique was used for the mapping of radon concentrations, annual effective dose (AED) for ingestion, and inhalation. Also, it is reached that the average of AED due to radon ingestion was 0.236 ± 0.020 μSv/year, while to radon ingestion were 0.015 ± 0.0001 nSv/year. Moreover, the range value of the total AED in unit μSv/year was 0.015–1.171, with an average value of 0.236 ± 0.020. Radon concentrations in most samples of water comply with the world-wide limits that are approved by WHO and ICRP which state that safe limits (0.5 and 0.4 Bq/L), respectively. Furthermore, all results of the total AED due to radon ingestion and inhalation assessment from radon concentrations in tap water samples collected from the primary schools in Najaf city when used as drinking water were within the global limitations of the WHO. One may conclude that from the results of the AED in children (7–12 years), there is no danger of radon concentrations in the case of human consumption.