Gee, I have a big collection of creepy little girls in movies, it's weird to read you couldn't think of any besides Regan and Damien boy! :))
 Ivy Templeton- Audrey Rose (even though Ivy is the reincarnation of Audrey Rose she acts as if she's possessed)

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Nicoletta Elmi as Emily in The Cursed Medallion
MarÃan Salgado as Susan Barnes in Demon Witch Child
 
Candace Hutson as Jessica Wade in Dolly Dearest

Brigitta Boccoli as Susie in Manhattan Baby

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Randi Allen as Cathy in Cathy's Curse

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There are others who aren't necessarily possessed but still fit in the creepy little girl category, such as: Jennie Westfield in Death Dreams (tv 1991), Carol Anne in Poltergeist, Flora in The Innocents, Anne in The Others, Jodie Foster as Rynn in The Little Girl Who Lives Down the Lane, Samara- The Ring, etc, etc. 😉