This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Making Fiends
"Puppies! Puppies! Puppies!" Charlotte meets her untimely demise again and again and again, and in each case, her last words are "Yay! Gumdrops!" Only one clone is aware it's a trap, and she finds out a little too late.
Turns out there were two Charlottes that didn't get eaten. Whether or not one of them was the original Charlotte is unclear.
Vendetta's Slasher Smile while she gives cookies to people in the episode where she is trying to be evil as Charlotte.
The end of Shrinking Charlotte, when the giant cat swallows the shrinking slug and one of the gianting squid.
The same episode had a rather creepy Leitmotif, with its table-rattling, two-note bass and eerie pads.
Basically the entire premise of the show could be this. A ten-year-old Enfante Terrible has taken over/traumatized a town using homemade monsters to the point that no one has the guts to go up against her. She is now hell bent on murdering an eight-year-old girl just because she wanted to be friends with her, and the only thing keeping this little girl from getting killed is her uncanny obliviousness and naivety. Black Comedy indeed.