1. The tetanus shot is to prevent tetanus, which is fatal somewhere between 10and 20% of the time. Tetanus spores are everywhere, not just on rusty nails. They're in the soil, in house dust, in some animal and some human excrement - everywhere. The spores will only become active if they are introduced into an anaerobic environment. Anaerobic means no oxygen. The human body is full of oxygen, because the blood carries it everywhere. The reason crushing, puncture and burn wounds are more susceptible to tetanus is because they are more likely to have problems with circulation to the injured areas, which would allow any tetanus spores to become active and start producing toxin.
2. It's not impossible for children to develop tetanus - it's just very, very rare because kids have such great circulation.
3. The medical community seems to believe that it might help if your last tetanus shot was not more than 10 years prior, because you will still have some immunity from your previous tetanus shots, so it won't take you long to mount a full immune response after a booster. (That's the theory.) If your last tetanus shot was more than 10 years prior, they will give you TIG and the tetanus shot. TIG is a blood product from people who are immune to tetanus. It provides antibodies to protect you while you're waiting for your body to start producing its own.
4. There were some studies done back in the 40's or 50's that showed that tetanus could be treated or prevented with high doses of Vitamin C. They were done on chickens and guinea pigs, though, IIRC. More recent research in humans has shown that tetanus can be treated with IV magnesium, but not all studies have produced the same results.