A foldable child support device includes a main frame unit, first and second frame units, and a seat support member. The main frame unit includes left and right frame portions. The first frame unit includes a first rod member, two arm members connected respectively to the left and right frame portions, and a first connector unit for connecting pivotally the first rod member to the arm members such that the first frame unit is foldable. The second frame unit includes a second rod member, a third rod member connected pivotally to the main frame unit, and a second connector unit for interconnecting pivotally the second rod member and the third rod member such that the second frame unit is foldable. The seat support member is mounted on the main frame unit, and cooperates with the first frame unit to define a seating area.