Final answer:
Tatum O'Neal is the youngest child to win an Oscar; she won at age 10 for Best Supporting Actress in the film 'Paper Moon.'
Step-by-step explanation:
The youngest child to win an Oscar is Tatum O'Neal, who won the Academy Award for Best Supporting Actress in 1974 for her performance in Paper Moon when she was just 10 years old. This distinction makes her not only the youngest to win the award in that particular category but also the youngest Oscar winner in any competitive category to date.