How do i make my back yard cooler for my dogs?

i have 4 dogs 3 chihuahuas (1 is 2 years old and other 2 are 1year old) and a Pittbull he is about 3 months old and they are still small how do i make the backyard cooler for them im about to start cleaning of all the weeds but i want to know how do i make it cooler for them they look thirsty and hot i try bringin them inside but they just want to go outside so i have to figure out how to make it cooler what will i need to make them more safe out their?