Just about any experiment that can teach children about air pressure is safe to perform. Below are 2 of them:
Why is the water floating?
Get a playing card or similar card, one that fits over the top of a small glass.
Fill the glass to the very top with water.
Put the card over the water.
Turn the glass upside down, with the card on it. The air pressure should hold the card onto the upside-down glass. If you have filled the glass to the top, then there should be no air in the full glass.
The pressure of the air is greater than the pressure of the water.
Is the water even liquid?
Get an empty bottle of Clorox; rinse it out well. Do not throw out the cap.
Close to the bottom, on the side of the bottle make a hole that is large enough for the end of a pencil.
Put a small pencil in that hole. Fill the bottle to the very top and close it tightly with the cap.
Working over a sink or bucket, pull the pencil out of the hole.
The water should stay inside the bottle. The air pressure should keep it from pouring out of the hole.
Are these beans magic?
Here is one other small experiment. It teaches a fact of botany, the study of plants.
Get a small plastic bag and a few dry beans.
Put a damp paper towel or tissue inside the bag, along with the beans.
Close the bag and hang it up somewhere. See what happens.
The beans should sprout. One sprout should grow downward, as though it were a root. One should grown upward.
Now turn the bag so that the side that was at the bottom is on the top. Again watch what happens.
Slowly the shoots that were growing one way should start to turn around and grow in the opposite direction. Plants detect the force of gravity, and it tells the plant which sprouts should grow down into the soil and which one should grow up towards the surface above the earth.