It fills things up, and can take the shape of whatever you put it into. Put a solid toy into a cup. See that it doesn't change shape. Water changes its shape but not as easily . We can show this by the fact that as you push your hand through water it is harder to do it fast, you can push the air away much faster.
Air is made of gas, the water is made of liquid and the toy is a solid.
Have the younger children give examples of each. We also said that air is pushing on us. Put the cardboard over the glass and then turn it upside down trick. The force of air pushing up is very strong. Air is heavy that is why it can push on us.