I was in Madrid and Barcelona from about July 8 to the beginning of August and it was really hot! I'm from Tucson, so I understand heat, but in Barcelona it was also humid. (I was in Granada and Cordoba during the same time period quite a while ago and it was in the upper 90's.) Air conditioning was not always available. I wore Exofficio skorts. They were longer than most shorts and had a skirt-like look about them. My favorite combo was a light beige skort with a white or black sleeveless shell and leather sandals. I felt conservative enough (I was with a group of students, so I had to dress the teacher role!) but was comfortable. I didn't notice people staring, though if they were, it was clearly to marvel at my great fashion sense
. Older French women still dress nicely and not like they are all in mourning, and my American friend who is married to a Spaniard and spends every summer in Granada dresses in colors and casual clothes (she likes color coordinated sports clothes like long shorts and matching t-shirts or culottes and t-shirts), so I don't worry about it very much. I don't wear very revealing or extreme clothes and try to look "put together"...like a casual Friday at work perhaps? How do people know if we are really old ladies who happen to look great or younger ladies who have spent way too much time in the sun?
Now as for your grandaughter, I have the same problem in trying to tell my young female students what to wear. I saw a lot of spaghetti strap and tank top numbers, but I can't say that I saw shorts cut quite to the heights of Tucson shorts...unless the girls were at the beach! Our kids have to carry their passports in carriers, so dresses become difficult. Jeans were ok...though hot. I am still trying to resolve the problem of how to handle the more revealing attire while touring cathedrals. I don't want to feel like a disrespectful American who cannot conform to dress codes, and I don't think most of my female students would ordinarily dress like they do informally when they go to church! When in doubt, we usually go slightly more conservative...even if our girls grumble!