When Aristotle theorized that all objects naturally want to be at rest, he did not realize that a force called friction is present whenever an object is in motion, and that this force opposes motion. Friction is caused by surface roughness at any point of contact with an object. Even in surfaces that appear smooth, there is roughness at an atomic level. If it's strong enough, this frictional force can cause moving objects to slow down or stop.
You can demonstrate what friction looks like, using a board that is at least two feet long and several inches wide. Place 4-5 small objects of different shapes and textures (e.g., an ice cube, a small wood block, a plastic toy) at one end of the board. Next, gently lift the board a few inches, until any of the objects begin to slide. Which one moves first? Keep raising the board gradually, until all the objects have slid down the length of it. Which ones 'stuck' the longest? Less frictional force was exerted on the objects that slid first. Ask your children why those objects had less friction.
Although friction can cause us problems, it's also a good thing—if there was no surface friction, we wouldn't be able to move. Friction is also what holds cars on the road when they go around a curve. Wet or icy roads and sidewalks are more difficult to travel because the water and ice reduces the frictional force.