Marie Antoinette adored children, and was often surrounded by them. I think it is safe to say this for two reasons, she grew up with many siblings and enjoyed the company once at Versailles (because she was so young herself), and also due to the fact that she could not/was not having children of her own in the years to follow.
Around the time when her sister in law the Comtesse d'Artois had a child, Antoinette took part (size of part to be disputed) in raising a peasant boy. The story is told by many, and even discussed in popular fiction books on the Queen. One book, which reader Marika has pointed out to me, The Queen's Confession, discusses the topic of Antoinette and her adopted peasant boy.
As the story goes (embellishments accepted): She was riding in the carriage and by an accident, a peasant boy fell under the hooves of the horses. Marie Antoinette was naturally very concerned, and had him brought to Versailles where he was taken care of by a nurse. After he was fully recovered from the dramatic accident, he was schooled, room and board on the crown. Ironically, years later this boy was said to grow up and become a dangerous revolutionary, very anti Marie Antoinette and her family.
I can not speak for the story being accurate. It is very possible it has been enhanced for effect over the years (undoubtedly so!). However, the fact that she took in a young boy is confirmed in a letter written by the Comte de Mercy dated September 17, 1776 in which he writes that "the boy is neither turbulent nor troublesome." His role was not big in Marie Antoinette's life, as her daughter was born two years later.