Portable Booster Seat

booster seat
An extra seat or cushion placed on an existing seat for a small child to sit on
An infant safety seat, also known as a child safety seat, a child restraint system, a restraint car seat, or ambiguously just as a car seat, is a restraint which is secured to the seat of an automobile equipped with safety harnesses to hold an infant or small stature people in the event of a crash.
(Booster Seats) Booster seats are not permitted in any Spaceships vehicle.
(Booster Seats) Are intended to be used as a transition to lap and shoulder belts by older children who have outgrown convertible seats (over 40 pounds). They are available in high backs, for use in vehicles with low seat backs or no head restraints, and no-back; booster bases only.

portable
of a motor designed to be attached to the outside of a boat's hull; "a portable outboard motor"
A version of something, such as a small lightweight television or computer, that can be easily carried
A small transportable building used as a classroom
a small light typewriter; usually with a case in which it can be carried
easily or conveniently transported; "a portable television set"