The greatest child star of all times is Shirley Temple. She was the Babe Ruth of child stars.
If you are talking about the greatest talent of all time, it would be Judy Garland.
If you are talking about the best child star to be a great talent as a child and as an adult and to have successfully made the transition between the two, it would be Jody Foster.
All, IMO, by the way.
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http://bible.cc/1_corinthians/13-11.htm