Hello Heather,
the Spanish law is quite strict with child car seats (not using the correct baby/kid car seat is fined) . The following car seats applied to the different kid/baby weight:
Car Seat Group 0+: 0-13Kgs
Car Seat Group 0/1: 9-18Kgs
Car Seat Group 2/3: 13-36Kgs
Car Booster Seat: Kids over 150cm
No kid below 150cm can sit on the front seat, even using the car belt.
5 point-harness belts are mandatory in both front and rear seats and almost every car has them.
Not using the car belt is also fined.
The following links might provide you with more detail information Car Seat Laws in Spain:
http://driving.drive-alive.co.uk/driving-in-spain.htm http://drivinginspain.blogspot.com/search/label/Belts%20and%20helmets%20etc.
Babytravelling (www.babytravelling.com) rents baby equipment including car seats.
Hope this information helps.
Regards,
Sol (Babytravelling)