How to keep dogs cool in the backyard during hot summer days?

I have 3 small size dogs (min pin, cairn terrier and a border terrier) that love to play in the backyard. Now the summer heat is a killer and although I keep lots of water bowls around and there's a big tree that offers shade and I even got them a little pool. However, I worry they still might be feeling hot and being at work I would not be able to keep a constant eye on them. Any suggestions? I read about cooling beds and cooling vests, are any of these any good? If so, any specific brands, etc? Any other tips?