Mona, meanwhile, doesn't notice things are real because she subconsciously thinks it's all in her imagination, whilst consciously believing it's real as part of said imagination. Charlie and Lily are the same, albeit to a lesser extent.
In fact, Mona is actually a vampire. Or rather, she has vampiric ancestry in her family. Her interest in vampires comes from a family "legend" she was told when she was younger, though in reality her heritage is active in her whilst it isn't in her parents (though it's not prsently active enough for her to have much real ability), which allows her to ignore the Weirdness Censor on supernatural or weird things because she's actually one of those things. Charlie and Lily are also unaffected to an extent by following her around so much.
Mona frequently mistakes innocent people, and even innocent objects, for being evil and dangerous. When it comes to Lily, Mona was the first child that she met on moving house, so naturally the two became friends- I've not seen every episode so I'm not sure if Charlie gets an origin story.
Mona's parents, teachers and other adults are oblivious to poor Mona's plight.