Objective Road safety of children has improved considerably over the past decade; however, there is still much scope to better protect these occupants. Ensuring adequate protection of children seated in the rear seats requires a greater understanding of how children are injured in side crashes. The purpose of this study was to enhance the knowledge of children’s potential injuries in side impact collisions by the investigation of two different child sizes seated in different positions in the rear seats. The injury metrics associated with the occupant-to-occupant or occupant-to-child restraint system interaction was also identified. Methods Finite Elements Method simulations were performed using a validated simplified vehicle model with fully deformable side components. Dynamic virtual tests according to the Euro NCAP Child Occupant Protection Protocol were performed considering three different scenarios in terms of occupants´ distribution in the rear seats. Simulations were conducted with dummy and human body models. The injury metrics associated with near-side and far-side positions and the occupant-to-occupant interaction were analyzed. Results The 10-year-old child seated in the far-side position presented higher injury values, especially in the head due to the contact with the adjacent child or child restraint occupying the near-side seating position. Similar responses were observed in human body and dummies models in terms of head and neck injury values. However, human body models showed an increase in the chest measurements. Conclusions This study highlighted that the potential injuries of a child seated on the rear seats in a side impact crash are likely to be increased by the presence of an adjacent seat occupant, especially when larger children were seated in the far-side seating location or a CRS was installed in the near-side seating position. Results offer insight into the occupant-to-occupant interaction and occupants’ size as relevant factors to improve children safety under side impact conditions.