A foldable child booster seat includes a seat base, a backrest, a support unit, and a tray. The seat base includes a seat portion having a seating surface, and a pair of armrests respectively disposed at left and right sides of the seat base. The backrest is mounted foldably to the seat base. The support unit is movably coupled to the seat base to permit height adjustment of the seat portion relative to the support unit. The tray is removably mountable to a selected one of the seat base and the support unit to dispose the tray in a selected one of a state of use and a stowed state.