A child booster seat assembly includes a base seat and an upper seat. A first seating area is formed on the base seat. The upper seat is detachably installed on the base seat. The upper seat can be installed on the base seat installed on an adult chair at a higher position or detached from the base seat and installed on the adult chair at a lower position, which allows a caregiver to adjust an elevated height of the upper seat easily. Furthermore, when the upper seat is detached from the base seat, the upper seat and the base seat can be used independently and respectively installed on two adult chairs for accommodating two children at once.