MsSage, I think the feed thing is what people need to understand the most. It may be a little to deap for kids though.
The cow has four stomachs. The first stomach, or rumen acts as a fermentation vate. It uses microbes or bacteria to break down the tough fiberous grass and hay and convert it into nutrients. We often think of bacteria as someting bad. If it were not for the good bacteria, we would not have butter, cheese, bread,(as we know it), vinegar or many other things.
The amazing thing to me is how it can convert simple protiens into the complex amino acids that we and all animals need. I believe humans need about nine different ones, a cow probably does too, but can produce some of them in her rumen. Cereals, and vegitables, for the most part do not produce some that we need, we balance that by eating meat, milk, eggs, etc. That is why we can say that the cow eats something we can not eat and changes into something we need.
To put it simply, microbes in the rumen feed on these simple protiens and cells, when these microbes die, they are digested to provide what is needed to nourish the body.