The experiences from several past radiological accidents (e.g., Fukushima Daiichi, Chernobyl, and Goiania) have shown that not only occupationally radiation-exposed adults but also children of all age groups can be affected by such an accident. Moreover, in asymmetric conflicts, children could possibly be affected by the explosion of a radiological dispersal device. Whole-body counting facilities all over the world are well prepared to measure children and adults but suffer in the case of newly born babies and toddlers owing to a lack of corresponding calibrations. A series of calibration phantoms representative of adult persons and larger children is commercially available or can be constructed from easily accessible materials, but for newly born and children up to 20 kg, only very few phantoms have been described which approximate the infantile body, and only very roughly. The aim of this work was to improve the current unsatisfactory situation. Two main steps toward achieving this aim included the compilation of anthropomorphic data for children and the construction of a modular set of calibration phantoms for the weight classes between 3 and 20 kg from simple, readily accessible, and cheap materials. The phantoms are made from commercially available freezer packs, which can be filled with a radionuclide solution. By using calibrations based on these purposely-built realistic weight class phantoms, internal radioactive contamination in children can be assessed more reliably than would be otherwise possible with the standard adult phantom calibrations.