There was (maybe still will be) a child seat available for the 2005 smarts, and described thusly: “Seat booster with backrest: For children weighing between 15 and 36 kg (ed. 33 lbs-79 lbs). With removable backrest and extra-large lateral support. Available in a choice of three colours.”
smart Part Numbers:
Seat Booster - Q0002617V002C80Y00
Backrest - Q0004663V002C80Y00
This seat was the largest of the three available.