Wheels and axles, such as those on carts, reduce resisting force by distributing it throughout the wheel or axle, and therefore make it easier to haul loads. You can demonstrate this to grade school age children using a Hall's cart or small toy car and a weight or some other object to be the load. First, try to push the load across a smooth surface. Explain to your children that there is some resistance, created by frictional force between the surface and the object. Next, set the load on the Hall's cart or toy car (you may need to use a rubber band to keep it on), and push it across the same surface. What happens? There is much less resistance, since the only part of the cart that is touching the surface is its wheels. The pressure exerted on the cart is distributed through the wheel and axle, rather than just between the surface and load.
You might want to look around your house for examples of wheels and axles. Thing like roller skate wheels, bicycle wheels, and the gears in clocks are all examples of wheels and axles. Doorknobs are another example. The rod (axle) that opens the door would be very difficult to turn if the knob (acting as a wheel in this case) was not connected to it. Not all doorknobs are round--some look more like regular levers—but they function in the same way. You might want to explain to your children how each wheel and axle works; have them identify the axle and what causes effort force to the wheel and axle.
The formula for finding the mechanical advantage of a wheel and axle is the ratio of the radius of the wheel to the radius of the axle. Thus, if you have a wheel that is 10 inches in diameter and an axle that is two inches in diameter, the radius of each will be half of that. The mechanical advantage will be a ratio of 5:1, or simply five.