Here's a nice clip of Susan Dey on the horror anthology series Circle of Fear (Ghost Story was the original title of the show) in 1973. This was Susan Dey's first TV appearance outside of The Partridge Family (though she had a small role in the 1972 feature film Skyjacked) and one of only two instances where she appeared in a horror-related project (the other being the TV movie Disappearance in 2002). Edit: Also (as a Susan Dey fan on Facebook kindly pointed out), Susan Dey's 1995 TV movie Deadly Love, about vampires and murder, should also rightfully be considered a horror-related project. My mistake for not including it!
In the Circle of Fear episode called 'Doorway to Death', Susan Dey plays big sister to two younger kids, played by Leif Garrett and Dawn Lyn. Luckily, Susan Dey already had a bunch of experience acting with children since she had been doing it on The Partridge Family since 1970!