Thyroid cancer is very rare in children. 131 I therapy after thyroidectomy is established in pediatric differentiated thyroid cancer (DTC). Pediatric DTC guideline is silent on the optimum amount of 131 I that could be safely and effectively administered to children who are more radiosensitive. Like adult DTC, children are also given 131 I therapy empirically based either on age or body weight. Pediatric DTC guideline recommends that patient-specific dosimetry is important in children. Still, due to the low incidence rate and the practical difficulties of dosimetry, it has neither been established nor adopted in routine practice. This review article aims to discuss current approaches of 131 I therapy in children and young adult patients with DTC and dosimetric data obtained by several investigators. Efforts are required to simplify dosimetric procedures and precise results, especially in determining lesion size. We prefer 3-dimensional dosimetry over planar dosimetry, where lesion size could be measured accurately. 124 I PET/CT-based dosimetry is expected to give accurate dosimetric results. The most challenging aspect is that no randomized controlled trials are available to compare the empiric 131 I therapy results versus dosimetry-based treatment outcomes in children and young adults. Suppose dosimetry-based 131 I therapy could be shown to have better outcomes, namely, successful ablation rate, better disease-free survival, and lesser treatment-emergent adverse events than empirical 131 I treatment. In that case, one can argue in favor of the former. Unfortunately, no convincing study is currently available. Thus, there is a need for a randomized control trial to settle this issue.