Yes, a child has won an Oscar. Tatum O'Neal became the youngest person to ever win a competitive Oscar in 1974 at the age of 10 for her role in the film Paper Moon.
The Academy of Motion Picture Arts and Sciences used to present an honorary Academy Juvenile Award for outstanding child performers. The first award was given to 6-year-old Shirley Temple in 1935. However, the award was discontinued in 1939.
While children haven't won Oscars for acting in recent years, they have been nominated for other categories. For example, Damien Chazelle was the youngest director to ever win an Oscar at the age of 32 for his work on La La Land.
