suffer. Here is a quote that shows this:

There are a lot of children in Afghanistan, but little childhood.

Here is another quote:

There is also discrimination against the Hazaras. Here is the full dialogue:

Assef's brow twitched. "Like pride in your people, your customs, your language. Afghanistan is like a beautiful mansion littered with garbage, and someone has to take out the garbage."

We can also talk about discrimination against women. There is a pervasive attitude that women are less than men; at times there is even abuse. 

The Hazaras are the ethnic group most discriminated against in The Kite Runner. They make up less than 10% of the population of Afghanistan (Pashtuns are the largest ethnic group), and are mostly Shia Muslim--also a minority in the predominantly Sunni Muslim nation. Their Mongol features also set them apart from other Afghan groups, and the Hazara have long been discriminated against for these reasons. Although the life of the Hazara became somewhat better during the Russian takeover, the arrival of the Taliban threatened the survival of the Hazara, who were the subjects of "ethnic cleansing" after being declared "infidels." According to Assef, who had personally slaughtered hundreds of Hazaras during the massacre in Mazar-e Sharif,

"Afghanistan is like a beautiful mansion littered with garbage, and someone has to take out the garbage... Ethnic cleansing. I like it. I like the sound of it."  (Chapter 22)

Discrimination as a theme is something that is evident throughout the novel in various guises, but perhaps it is most clear in the way that Assef treats Hassan and talks about him. Consider how he presents Hassan in Chapter Five in a way that clearly indicates how discrimination is linked with racism in his mind:

Afghanistan is the land of Pashtuns. It always has been, always will be. We are the true Afghans, the pure Afghans, not this Flat-Nose here. His people pollute our homeland, our wantan. They dirty our blood.

Assef clearly refers to the way that Hassan, because he is not a Pashtun, is something of a corrupting influence that threatens the supposed purity of Afghanistan. As a result, he clearly behaves in a way that expresses his hatred of Hassan and of all who come from the same racial stock. A good thesis statement therefore might be:

Discrimination in The Kite Runner is reflected through the character of Assef and his treatment of Hassan.

This would allow you to use the above quote to support your point and also to look further at the way in which Assef discriminates against both Hassan and also Hassan's son. Hope this helps!