Marie Antoinette was the focus-centre of many rumours and royal gossip. One of the rumours that surrounded her before the revolution was that she evidently adopted a peasant boy!
Reality was that Marie was very fond of children and wanted to help a poor peasant family. She provided money for the education of the peasant boy, Jacques, and helped his family out financially. Marie was very interested in the boy's upbringing because - at the time - she had no children of her own. However, that she "adopted" him is a wrong term. If she had adopted him then he would legally be her son - something that definitely could not be allowed for a Queen of France.
Instead she would follow the boy's life with interest. When she had her own children her attention towards the little boy naturally declined but she never stopped checking up on him.
So, to sum up:
Marie Antoinette did not adopt a boy but paid for his education and aided his family with their financial problems. She most likely did it because she had no children of her own combined with her great love of children.