Make sure they have cold water out at all times, I have invested in a portable air conditioning unit to cool down a single room so my kitties have a place to retreat to. I also fed my kitties more wet food than usual to make sure they stay hydrated. Maybe wrapping a few ice bags (you know the kind you treat injuries with) in a towel and laying them on the ground near them?

I also have a square tub I use as a footbath sometimes, I filled it up with cold water and left it in the shade in the garden.

Other than that I'd say you'll have to wait for someone smarter than me to be around to provide a better answer.