I think what you are looking for is a familiar analogy, or something the troop can understand by experimenting. There are two main reasons why one star could be brighter than another star. A good way to explain these two reasons to kids would be to compare the stars to light bulbs or flashlights. They will be able to see for themselves that a dim flashlight right next to you can appear brighter than a powerful one further away. Stars look different colors because they are different temperatures.
Hotter stars are white or even blue and cooler stars are orange or red.
Think about an electric stovetop. When it is turned on, the coils start to glow. At first they turn red, then orange and then brighter orange as they heat up. The scout leader can show them a candle flame to demonstrate. It actually has different colors, all corresponding to different temperatures.