As far as abstaining, you certainly don't need to. You can go to IMDB and get a good idea of how successful their careers were. Still, it's your choice.
Jean-Pierre Leaud wins (I would definitely have voted for Rooney in that matchup).
Next,
Catherine Deneuve vs. Judy Garland
Catherine Deneuve: Child: 3 (Ladies Man) Adult: 87 (The Umbrellas of Cherbourg, Repulsion, Belle de Jour)
Judy Garland: Child: 13 (The Wizard of Oz, Love Finds Andy Hardy, The Broadway Melody of 1938) Adult: 24 (Meet Me in St. Louis, Easter Parade, A Star is Born)
Oops, I nominated Catherine under false pretenses, thinking she was only 16 when she did Umbrellas and Repulsion. I didn't know she did a movie when she was 3.
Doesn't really matter since my vote would be Judy Garland against almost anyone.
Thanks for clarifying Brook. I don't think it'll matter, but I was a little confused since she only did 3 films under 18, and 2 of them on IMDB are still waiting for 5 votes. A lot more successful career as a child actor than most of us, but not really compared to the other entries in the tourney, at least IMO.