What is the tradition in Spain regarding tipping in restrooms? Do they have attendants? If so, what is the usual tipping
Only in night clubs from my experience.
A euro coin would be a normal tip.
I was in Spain last month. I do not remember attendants in the toilets. You may just run into toilets where you have to pay .50 to 1.00 euro coin to enter.
It is common in Spain to not have public restrooms. You may walk into a bar or restaurant for bathroom needs without a purchase. Do not recall any attendants. Some bars had a key but were more than willing to hand it out. Say Thank You or buy something and the Tapas start coming in smaller communities, it is up to you. Generally, tapas are not free in large cities and tourist areas.
Spain doesn't have the same tipping culture as US.