There's really no way for kids to understand the scale on paper. If you were to print an image of our Solar System to scale, the planets would be invisible.
What students need is something more tactile, a way to physically experience and grasp the actual scale. To accomplish this, you'll need to stretch things out.
So how do you get your kids out of their desks and venturing through the cosmos? First, you'll need a good long nature trail or walking path to do it safely. I prefer asphalt because it looks more like the darkness of space. When you have chosen your canvas, you'll need to print your planets to scale depending on the distance you have to work with. Starting with the size of your sun, use the chart below to plan your Solar System Walk. You'll probably want to use chalk to draw your Sun since it's so large in relation to the planets. Planets can be printed out and taped to the pathway, or you can simply use sidewalk chalk to draw them too.
How far do you need? Well that depends on whether you want your planets to look like planets or pinpoint dots. For a better visual impact, I would work with a scale that gives you an earth that is .3" to .5" across and a Jupiter that is around 5" across. This gives you a Complete Solar System laid out to scale in about 2.5 miles. For a more practical length I would suggest stopping at Saturn to keep your walk at about a mile total length.
A little longer than you had thought, right? That's the idea here, even us well-educated adults fail to have a real sense of scale when it comes to the Solar System in which we live. So get on your walking shoes and enjoy your learning experience while walking on the surface of planet number 3!
Type in the size of your Sun and calculate.