Upvote:2
According to https://www.busbud.com/blog/travelling-spain-bus-need-know/ your answer depends on what company and class you'll be traveling.
The Premium Class Provides Baby Seats.
Regardless, I would check with your travel agent to confirm seating for your child, and make sure they know that you are traveling with a baby so preparations can be made.
Even if you don't need it on the coach, might you need it for driving once you reach your destination?