As from 1st October, children under 1.35 metres tall must travel in the rear seats of a car in Spain. They must also use an approved child seat. There are just 3 exceptions.
The exceptions being that 1) All the rear seats are already occupied by children under 1.35 metres 2) the vehicle is a 2 seater (ie has no rear seats) and 3) it is not possible to instal all the necessary child seats.
Full details in the Dirección General de Tráfico article below.