For me (off the top of my head and in no particular order):
Natalie Wood in MIRACLE ON 34TH STREET
Patty Duke in THE MIRACLE WORKER
Patty McCormack in THE BAD SEED
Tatum O'Neal in PAPER MOON
Jackie Cooper in THE CHAMP
Justin Henry in KRAMER VS KRAMER
Claude Jarman Jr. in THE YEARLING
Margaret O'Brien in MEET ME IN ST LOUIS
Macauley Culkin in HOME ALONE
Natalie Portman in LEON: THE PROFESSIONAL
Natalie Wood in MIRACLE ON 34TH STREET
Patty Duke in THE MIRACLE WORKER
Patty McCormack in THE BAD SEED
Tatum O'Neal in PAPER MOON
Jackie Cooper in THE CHAMP
Justin Henry in KRAMER VS KRAMER
Claude Jarman Jr. in THE YEARLING
Margaret O'Brien in MEET ME IN ST LOUIS
Macauley Culkin in HOME ALONE
Natalie Portman in LEON: THE PROFESSIONAL