My adult cats seem to manage quite well in the heat but I having kittens in a heatwave isn't funny. There are plenty of ways to keep small kittens warm but we never think of them being too hot. The best solution I've found is to freeze water in lemonade or coke bottles and then wrap a thin towel round the whole thing and put it at one end of the pen/bed/box. Just as kittens can move on or off a heat mat, they will (and do) position themselves according to need in relation to a cool source. I've found this low-tech solution gentler and more flexible than fans or aircon units.