Objectives: Replacing conventional round intraoral collimators with rectangular collimators provides a considerable radiation dose reduction in adult patients. This study aimed to determine the radiation dose reduction via mathematical phantom when converting from round to appropriately sized rectangular collimation in children ages 5 to 15 years. Method and materials: Virtual full mouth series (FMX) were simulated using a commercially available radiation dose software. This software is designed to calculate patient radiation doses from x-ray exams for various age pediatric and adult mathematical phantoms. For this pediatric study an 18-image FMX was simulated for the 15-year-old and a 12-image FMX was simulated for the 5-year-old and 10-year-old pediatric phantoms. An area of 12.0 to 16.8 cm2 represented rectangular collimation, while a 20.4 to 31.7 cm2 area represented typical round collimation. Results: Effective doses decreased in all ages by nearly 60% when switching from 31.7 cm2 round to 12.0 cm2 rectangular collimation. Reduction in absorbed doses to the thyroid (70% to 73%), salivary glands (62% to 78%), and active bone marrow (60% to 62%) were also noted when switching from the largest to smallest collimation. Conclusion: This study suggests the use of rectangular collimators provides clinically relevant dose reduction for pediatric patients, even when altering from smaller round to rectangular collimation with equivalent beam quality, and this information can be utilized in all dental practices. Keywords: American Dental Association, full mouth series, intraoral x-ray collimator, mathematical phantom, pediatric x-ray dose, rectangular collimator, round collimation