Why are only children chosen for the Hunger Games?

Question

I was wondering as to why only kids from each District are chosen to enter the Hunger Games instead of any of the adults?

Answer 1

From the first book:

(...) The Treaty of Treason gave us the new laws to guarantee peace and, as our yearly reminder that the Dark Days must never be repeated, it gave us the Hunger Games.
The rules of the Hunger Games are simple. In punishment for the uprising, each of the twelve districts must provide one girl and one boy, called tributes, to participate. The twentyfour tributes will be imprisoned in a vast outdoor arena that could hold anything from a burning desert to a frozen wasteland. Over a period of several weeks, the competitors must fight to the death. The last tribute standing wins.
Taking the kids from our districts, forcing them to kill one another while we watch—this is the Capitol’s way of reminding us how totally we are at their mercy. How little chance we would stand of surviving another rebellion. Whatever words they use, the real message is clear. "Look how we take your children and sacrifice them and there's nothing you can do. If you lift a finger, we will destroy every last one of you. Just as we did in District Thirteen."

(Emphasis mine).

Answer 2

In short: Taking children is a more frightening punishment for the defeated districts. From the interview with Suzanne Collins at: http://www.thehungergames.co.uk/about_the_author

A significant influence would have to be the Greek myth of Theseus and the Minotaur. The myth tells how in punishment for past deeds, Athens periodically had to send seven youths and seven maidens to Crete, where they were thrown in the Labyrinth and devoured by the monstrous Minotaur.

Even as a kid, I could appreciate how ruthless this was. Crete was sending a very clear message: “Mess with us and we’ll do something worse than kill you. We’ll kill your children.” And the thing is, it was allowed; the parents sat by powerless to stop it. Theseus, who was the son of the king, volunteered to go. I guess in her own way, Katniss is a futuristic Theseus.

Answer 3

I'm pretty sure the exact reason as to why only children are chosen is given in the book. I will research that and add it to this answer, should I find it.
Until then, maybe this quote from the Hunger Games Wiki may suffice:

Every year since the rebellion, the Capitol forced 24 children into the arena (...) in order to both entertain the Capitol citizens and remind the twelve districts how completely at the Capitol's mercy they are.

Like I said in my comment to the question, I think the fact that the tributes are children is just the icing on the "we can do whatever we want to you" cake, to make it that more evil.

As to the question in the comments, regarding volunteering, The Hunger Games Wiki has to say this:

By rule, once a person's name has been chosen to become a tribute, another eligible boy or girl may volunteer to take their place. (emphasis mine)

With "eligible" meaning "between the ages of 12 and 18" (taken from the same wiki page).

(Out of universe: The Hunger Games is a Young Adult trilogy. I can see how it would take away from that to let the teen-protagonist fight grown men and women. The way it is, it just fits the genre that much better.)

Answer 4

In the Epilogue of The Ballad of Songbirds and Snakes a reason for this is alluded to:

“It certainly supports her view of humanity,” said Snow. “Especially using the children.”

“And why is that?” asked Dean Highbottom.

“Because we credit them with innocence. And if even the most innocent among us turn to killers in the Hunger Games, what does that say? That our essential nature is violent,” Snow explained.

“Self-destructive,” Dean Highbottom murmured.

Answer 5

Because it proves a point about the need for control

In The Ballad of Songbirds and Snakes, we learn about a character called Dr. Gaul.

Dr. Gaul is as close to sadistic as possible:

• She instilled the Hunger Games
• She created the Muttations
• She is happy to kill or torture even Capitol children to prove a point
• She has a firm belief that humanity is inherently violent and in need of control

During Coriolanus Snow's youth and his encounters with Dr. Gaul, he learns that she believes that human beings have a tendency to turn savage, unless control is applied:

“Your emotions are running high,” Dr. Gaul told the class. “I understand. I do. But you must learn to harness and contain them. Wars are won with heads, not hearts.”
“I thought the war was over,” said Livia. She seemed angry, too, but not in the same way as Sejanus. Coriolanus guessed she was just peeved about losing her strapping tribute.
“Did you? Even after your experience in the arena?” asked Dr. Gaul.
“I did,” interjected Lysistrata. “And if the war is over, then technically the killing should be over, shouldn’t it?”
“I’m beginning to think it will never be over,” conceded Festus. “The districts will always hate us, and we’ll always hate them.”
“I think you might be onto something there,” said Dr. Gaul. “Let’s consider for a moment that the war is a constant. The conflict may ebb and flow,- but it will never really cease. Then what should be our goal?”
“You’re saying it can’t be won?” asked Lysistrata.
“Let’s say it can’t,” said Dr. Gaul. “What’s our strategy then?”
Coriolanus pressed his lips together to keep from blurting out the answer. So obvious. Too obvious. But he knew Tigris was right about avoiding Dr. Gaul, even if it might bring praise. As the class chewed over the question, she paced up and down the aisle, finally coming to a stop at his table. “Mr. Snow? Any thoughts on what we should do with our endless war?”
He comforted himself with the thought that she was old and no one lived forever.
“Mr. Snow?” she persisted. He felt like he was the rabbit being prodded by her metal rod. “Want to take a wild guess?”
“We control it,” he said quietly. “If the war’s impossible to end, then we have to control it indefinitely. Just as we do now. With the Peacekeepers occupying the districts, with strict laws, and with reminders of who’s in charge, like the Hunger Games. In any scenario, it’s preferable to have the upper hand, to be the victor rather than the defeated.”

And later, Snow realises her beliefs:

“I don’t know,” said Coriolanus. “It’s like . . . you know how she’s always torturing that rabbit or melting the flesh off something?”
“Like she enjoys it?” asked Sejanus.
“Exactly. I think that’s how she thinks we all are. Natural-born killers. Inherently violent,” Coriolanus said. “The Hunger Games are a reminder of what monsters we are and how we need the Capitol to keep us from chaos.”

an elaborates further to Dean Highbottom:

“It certainly supports her view of humanity,” said Snow. “Especially using the children.”
“And why is that?” asked Dean Highbottom.
“Because we credit them with innocence. And if even the most innocent among us turn to killers in the Hunger Games, what does that say? That our essential nature is violent,” Snow explained.

Which is why he becomes such an avid supporter and expounder of the Games.

Answer 6

Basically because its a book oriented to a teen audience, so they pick children so the audience objective are able to empathize" more with the main characters.

Also, you shouldn't dwell too much on the logic of the society of The Hunger Games, it's a society rigged to destroy itself in civil war against the Capitol, not to endure for centuries. Why else pick 2 children each year? There are different ways of oppressing a society, but making human sacrifices to the death usually doesn't work (although it worked for the Ancient Roman society).

It also may be a highly stereotyped and compartimentalized mock of the society in the USA.