It's already done at many schools for fundraisers. Things like having the kids sell chocolates or magazine subscriptions. Also girl scouts with their cookies.
Much of the time it's the parents doing the pushing, and the kids in tow for the "do it for the children" factor. The kids probably don't really know what's going on, and don't have to take direct responsibility for any of it.
People buy that stuff just to help out kids though. Going door to door selling a 'magical candy bar' that can radically change someone's life for eternity would be much harder. I guess part of that is also conviction in what you are selling.