>>> FROM HERE <<<
Seatbelts
It is compulsory for all in the car to wear seatbelts, both front and rear where fitted. The driver is responsible for any fines where passengers are not wearing an approved belt. Children under 12 years of age are not allowed in the front seats (unless they are over 150 cm or 4 ft. 9 ins, then they can unofficially get away with it. It is apparently to do with being secure in the safety belt). Also, if seated in the back, the belt must fit correctly, or a special “raising seat” must be fitted.
** Interesting that the driver is responsible for passengers wearing seatbelts. We tell our friends they need to belt up as it's amazing to me how some won't. We have a friend who justs pulls the belt across to appear she's belted in. Why should my husband who is doing a favour by offering to drive that day so someone else can enjoy a drink, be responsible for them as adults ? 
>>>FROM HERE <<<
Children under 12 are not permitted to travel in the front seat unless restrained in a suitable car seat. All children up to the age of 12 or 150 cms should use a car/booster seat
THE BEST INFO I READ WAS from Get Spain
Up to 9 months and up to 10 kg, a carrycot must be used laid across the rear seat and secured with the car's safety belt or any specially approved attachment.
From 10kg to 13kg and up to 2 years of age, a child's car seat fitted in the rear with the child facing rearwards.
From 9kg to 18kg and from 9 months to 3 years old, child seats facing frontwards or rearwards , fitted in the rear of the car and secured as detailed in the seats instructions.
From 15kg to 25kg and 3 to 6 years of age, use a seat adaptor (elevator) to lift the child up to fit comfortably with, if necessary, a CE-EU approved seat belt adaptor.
From 22kg to 36kg and 6 to 12 years of age, use a seat elevator so that the child is comfortably protected using the seat belts.
Children up to the age of 12 are not allowed to sit in the front seat.