Having googled about this, I've come up with conflicting statements but the AA state that, in Spain:
"Passengers/Children in cars: Children up to the age of 12 and measuring less than 135 cm travelling on the front seat of a car must be seated in a child restraint system adapted to their size and weight. Children measuring more than 135 cm may use an adult seatbelt."
As the AA website is the most professional of those that came up in my google search, I'm taking it that they are correct? My ten year old is well over 135cm, so presumably he can sit in the front seat, using the installed seat belt?