Obviously when you're organizing your Cub Scouts fundraising effort, you want to get the boys involved as much as possible. However, kids of a very young age probably won't really know what you're talking about when you try to explain the monetary aspects of Cub Scouts fundraising, and in addition talking about money will bore them to tears. You can lighten things up a lot by simply explaining to them that Cub Scouts fundraising is there to help them do all the fun and exciting things they enjoy doing, because those things cost money. Mention specific activities that they love, and let them know that their fundraiser is going to be a direct help in them continuing to do those things.
When you choose an Cub Scouts fundraising idea, it will definitely be helpful to make sure that the boys understand what the product entails and why it's a good idea. This will be easier with some products than with others - cookie dough or chocolate probably won't take much explaining, as you might imagine. But if you have chosen a Cub Scouts fundraising idea that's more geared toward an adult market, like coffee or green products, you'll want to make sure that the kids understand that it's important to have a product that grown-up will be interested in buying.
Keeping the fun in your Cub Scouts fundraising effort is the most important part. Kids have a highly contagious energy, so if you can get them excited about their fundraiser, great success is sure to follow.