An evil little girl is one of the most favorite tropes of a horror movie genre. Just to name the most iconic ones – The Exorcist, The Shining, The Ring – they all feature little, pre-teenage girls as channels of evil.
I don’t think it’s accidental. What drives a little girl? Nothing that would make sense in our adult world. She’s interested in nature and ponies. One day, when she hits puberty, she will discover the money-sex-power triad and things will never be the same. The evils of a teenage girl, and really of any adult, are boring and predictable: meanness, greed, obsession with hierarchy, trading on her sex-appeal. We do not find any of this scary, because we understand where she’s coming from. We are all operating in the same mutually-understood, dominant social framework.
Do you think these girls care about their future career or earning potential or meeting a nice guy?
But it is the pre-pubescent girls (along with mentally sick) that really scare us in horror movies. We don’t know what drives them. ‘Nature and ponies’ realm is alien to us, adults. Those residing in that realm are scary to us because we don’t know how their mind works. ‘Nature’ and ‘ponies’ are signifiers of things that do not fit into our modern values system of ‘money’ and ‘profit’. Because they don’t yet speak the language that any of us speak, because they don’t yet understand and don’t really care about ‘how business is done, how the world works’, we realize that we can’t reason with such a person, and thus can’t cure her possession. At least not until she hits 13, when we all can exhale with relief and welcome her to our ‘normal’, real world order. (Now, I’ve heard, from many parents, that puberty is even worse than childhood, but in the context of what I’m discussing here – horror movie tropes – children, girls especially, are scarier than teenagers).
Take the twins from The Shining or a girl from Hereditary. Do you think any of these girls care about their future careers or earning potential or meeting a nice guy? This is what’s scary about them: with their irrational behavior they telegraph the existence of a world that operates with notions that don’t produce profit or pleasure. Thus the carrier of this bad news, of this unprofitable sentiment – a girl – is, implicitly, a threat to our normal way of life. She’s more horrifying than a monster, because, and I will repeat myself, we know what drives a monster – he or it needs our flesh or brain to survive; a monster is not really scary because its behavior can be rationalized and thus harnessed. But a girl – who the fuck knows what’s on her mind. She’s on par and perhaps even surpasses, in her terror factor, the silent goons from Halloween and Friday the 13th.
The films featuring the possessed girls, however, never choose to explore this very ‘nature’ and ‘ponies’ aspect of girlhood. Instead, the plot usually revolves around curing them from evil ailment or sending them back to hell, and saving the main protagonist – an adult. One day, a script will be written where such a girl ‘cures’ an adult of his, shall we say, misconceptions. It’s a novel concept, but some are already beginning to chip away at this, like Game of Thrones.
Left with Balls 
I think girls are used as vessels of evil and demonic possession mainly for the shock value: most boys are already vessels of demonic possession by default.
I think boys understand hierarchy and ‘winning’ – the notions the we all operate with as adults – from the early age. Thus, it’s easier for us to make sense of a boy’s behavior and motivations than a girl’s. Thus, they’re less scary, in my opinion. Although the boy from Omen, was pretty scary too.