"Wear what's in your closet" "Have a layering system" - that is such good advice! When we did our first camino with eight kids we wore whatever we wore at home. And for some of them that did mean denim shorts! Our one splurge had been decent raingear for everyone - but it was still something we already had for our hiking at home.
I think I'll join Annie for my next camino because I could do without getting wet;-) From Sevilla to Merida it rained every day, sometimes all day. We were sodden - and after that experience the kids saw the sense in my insistence that we keep one complete set of clothes for "after walking" and not let it get wet, even if it meant putting on damp walking clothes in the morning because they had not dried overnight. It was so good at the end of each day to have something dry to change into, especially on the day the showers were cold!