How can explain how steam engine works and what it is to children? (James watt invention)?

I would begin by explaining that the earliest industry used water falling down and turning a water wheel and thus grinding grain into bread, etc. Then I would talk about the process of blowing up a balloon, and how that is kind of what boiling water/steam does when it pushes on a piston in a cylinder. And blowing a spit ball through a soda straw is also a similar process. The boiling water/steam can be made to push a wheel and thus some useful work can be accomplished, like the water wheel, but with boiling water providing the power rather than falling water.

Have them blow through a straw to understand forced and strong air pressure. Then aim the straw at one of those pinwheel toys. {Or maybe also blow bubbles in their milk.} After that, they should get the idea of steam under pressure pushing on _____ tom make things (axles and wheels) move./

water/steam turbine

Tell them to picture a ball with a hole, then have the ball slide over a stick. You could spin this ball with your hand. Put spoons on the outside of the ball. Eventually, humans would design things that had humans and animals turning their version of this device to draw water from under the ground, for instance. Tell them how the wind, or your breath, could also drive this machine. As long as 2 thousand years ago, people started to use hot water to drive devices that they used as toys, or oddities. I am familiar with ancient technology, but I cannot describe how to tell children the rest. Ask in the science/ engineering category maybe.