Bear in mind, also, that the child in question is Maggie, who's been exposed to the supernatural from birth, and the family she's been with is the Carpenters. While in the Dresdenverse, there may be an active or passive conspiracy to veil the supernatural, so to speak, we're looking at a very aware subsect within children in general.
Maybe all kids in the DV have a sense of "bad things", most kids grow up scared of monsters, things that go bump in the night, hands under their bed, or having imaginary friends. The Carpenter family, including Maggie, are that much more aware that these things exist and have names. Kids are told, from a young age, that it's just their imagination, or it's just the wind, or it's just that tree tapping on their window.
It could be that the kids that hunt Maggie are really just bullies to anyone else, however Maggie(and probably Harry and Mouse) create a sort of resonance that amplifies and encourages the haunts to track and attack her.
Just my 2c.