A Question about the little girl in red dress in Schindler's List

The girl in the red dress in the film is different to the girl in the red dress

The Girl In Red in real life was Roma Ligocka, who survived the holocaust and wrote The Girl in the Red Coat.

She has stated that Schindler's List was the inspiration for her book, as she saw the little girl in red in the film and was reminded of herself. To quote from her:

"When I saw the film it was like a flash of lightening," she explained. "All of a sudden fragments, bits and pieces from my past, grew together to become one."
...
"I knew at the age of two that I was not supposed to live, that I would be killed sooner or later," she recalled. "I wanted to live so badly and my only thought was that I needed to survive."

The Girl in Red in the film was probably a fictional character (although it could have been a retelling of her story, though that seems unlikely), whose red coat was the only flash of colour in the film. To quote from the Wiki (which references Steven Spielberg: A Retrospective:

Spielberg said the scene was intended to symbolise how members of the highest levels of government in the United States knew the Holocaust was occurring, yet did nothing to stop it.

"It was as obvious as a little girl wearing a red coat, walking down the street, and yet nothing was done to bomb the German rail lines. Nothing was being done to slow down ... the annihilation of European Jewry," he said. "So that was my message in letting that scene be in color."

Incidentally, the actress who portrayed the fictional girl in red, Olivia Dabrowska, was traumatised for years after the film, when she broke a promise to Spielberg and watched the film when she was 11 (he wanted her to wait until she was 18).

I did not read the book, but I think that the kid in the movie is simply a way to impress the viewer: an innocent person whose only fault was being born Jewish, was brutally killed in the concentration camp by the inhumanity of Nazism.