What's absurd (or, at least, one of the things that's absurd) about The Boy in the Striped Pyjamas is not that there was a 9-year old in the camp, but that a 9-year old in the camp had been given a striped uniform and sent off to work.
The uniforms given to the camp inmates were not tailored, and frequently ill-fitting as a result. If you look at the famous Soviet photograph, of children in Auschwitz, you can see how the uniforms hung off them:
These children, who were all in Auschwitz at the time of its liberation, had not been given uniforms. Uniforms were given to them, for the purposes of this photograph, by their Russian liberators.
So why were little children in Auschwitz?
The overwhelming majority of children who turned up at Auschwitz were selected to go to Birkenau. Some children, whose appearance was such that they might have passed as being over the age of 16, were selected to go and work. But some little children were selected to go to the concentration camp at Auschwitz (Auschwitz I), even at a late stage of the war, and based upon the concern that the Red Cross may wish to investigate the camp. There, the were put in Block 31 of sector BIIb.
Many of those children were brought from Theresienstadt, and the Nazis set up a familienlager in Auschwitz for this purpose. Children were kept with their parents, (obviously) not given uniforms, and were treated better than those outside of the "family camp". Most specifically, they got to hold onto their luggage, and their heads weren't shaved. Some of those children survived the war, although the precise circumstances under which they did so is a matter of historical discussion.
For more information, see David Cesarani, Final Solution: The Fate of the Jews 1933-49, p685 - and his references there (specifically Nili Keren's article in Gutman's Anatomy of the Auschwitz Death Camp). One of the children in the family camp was Otto Dov Kulka, who later wrote Landscapes of the Metropolis of Death.