I always hear older British celebrities waxing nostalgic about how terrifying Doctor Who was when they watched as a child. The Doctor has a long history of facing some creepy threats, but I can't imagine facing some of these as a child. The Empty Child/The Doctor Dances two-parter is a great example. I saw this as an adult and Jamie is frightening in his gas mask desperately searching for his "mummy."
Fortunately, the Doctor saves the day and the show introduces everyone's favorite omnisexual immortal Cap'n Jack Harkness!