This will my first summer with my first Yorkie. But I used to keep ferrets and they overheat very easily, I used to have an electrical fan near their cage and spray them with cold water as often as I could. If they got really hot I would dip them in the sink! Maybe you could set a little pool/tub of water up for them! If not outside then somewhere indoors with a plastic sheet underneath! Hope this helps.
Also I have a little clip-on battery fan I use on my baby's pram, if you have a crate this may be a good idea!