The rules in Spain are the same as UK. Children must have their own car seat. I had to have a booster seat for my 8 year grandaughter last year and a child seat for my 5 year old grandson. Children under 135cm must have a booster seat.
This from the Get Spain website:
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.
. There are hire shops which will rent out a child car seat and I have definitely seen such shops in Torrevieja but can't remember where. I'm sure they'll have the same type of shops wherever you are.