The most important thing to do is stay hydrated. Drink lots of fluids, especially water. Stay away from alcoholic beverages. And even though an ice cold beer is so refreshing on a hot day try not to overdo. Caffeinated beverages and soft drinks should also be kept to a minimum. They strip you of fluids..
Eat foods that will help hydrate you. Fruits and vegetables are good. Salads are good meals during hot weather. Meats heat you body as you are digesting them so find other sources of protein. Eat sparingly and often. You will get your nutrition but not put your body to work trying to process a large amount at a time.
Air conditioning is wonderful. I have no idea how we lived without it before it became a necessary appliance. Staying out of the heat is the best way to stay cool. The problem is that the children need to be out in fresh air getting some exercise.
I have some ideas that can help you be out in the heat and stay cool at the same time. I do caution you to pick times of the day when the sun is not at its worst. Also sunblock will protect delicate skin.
If there is no swimming pool nearby invest in an inexpensive wading pool. If you fill it in the morning and let it sit in the shade it will provide a cooling activity without that shock of really cold water.
Water balloons are fun. A tub full of them can make many cooling games. Each child can take a water balloon and throw it up in the air. When it lands it will splat. The biggest splat is the winner.
Play dodge ball with water balloons. When they hit the person being thrown at, they will break and wet him/her down. It is a cooling thing.
A version of hot potato can be played with a water filled balloon. Before filling the balloon with water, gently prick it a few times so the water will slowly leak onto each player. Sit or stand in a circle and gently toss the balloon around to each person. Keep going until the water finally empties completely. Each person will get wet and that will cool her/him off. I find it fun to have more than one balloon circulating at a time if there are enough children.
Ice cubes melting on the sidewalk is fun to watch. See whose ice cube melts first and that person will get to hand out a cool treat.
Finger painting is such fun. You can make you own paints. Take 3 teaspoons sugar, 1/2 teaspoon salt, 1/2 cup cornstarch.and 2 cups water. Combine the ibgredients in a small sauce pan and heat until the cornstarch thickens. Let it cool and pour into small containers. Baby food jars are good. Add a bit of food coloring to get the colors you want. This is not meant to be edible but is non-toxic.
Find a good place outside for the children to work. Wet the paper they will paint and let them create. After every little artist is finished, take them into the yard and turn the hose on them. They will have fun, get clean, and be cool.
Children love squirt guns. Fill them with water. Have one child be "it". That child will be blindfolded and hold the squirt gun. He will call Marco. One of the others will answer with Polo. "It" will aim and fire at the sound. When she hits the Polo person they trade places.
Remember bobbing for apples? Float the apples on the water in the pool. Each child takes a turn to hold her hands behind her back and try to capture an apple in her teeth. It is a fun contest and keeps the children cool.
Various items from the house are needed for this one. Take each item and ask the children whether it will float or sink. Then let the children take turns conducting tests on the objects.
Picnics are fun. Unfortunately when it is this hot a picnic can be so uncomfortable. Why not have it indoors? Pack up the food and drinks and sit on the floor and have a blast. I prefer to use a floor that is easy to wash in case of spills.
There are other things to keep cool. I would love to learn about them.