The community is a small one about an hour away from the closest town and rural. The kids are siblings, a boy and girl. The world I set this in has people with magic abilities, but magic is completely hidden from outsiders, so this community has no idea that the kids have magic.The kids' family was the founding family in that community and their parents died in a freak accident. I need a reason for the community to be really hostile towards these kids.
I'm not sure if your question is on-topic because Worldbuilding generally focuses on physical elements of worlds (the creatures, the landscape, the infrastructure, etc) as opposed plot elements. You may find better answers on the Writers Stack Exchange.
First, is this the right choice?
It is important to make balanced characters. The plot is ruined if the protagonists are powerful enough to solve every conflict right away. The lesser-known side of the "balanced" spectrum involves characters too weak or in situations too dire to identify with. By making a "pity-party" scenario in which these characters just lost their parents - and now everyone hates them - you may lose some readers.
On to the Question
Suspicion of Involvement
Perhaps the "freak accident" left both kids unscathed when it should have killed the entire family - such as an explosion. Some person decides that the kids might have had involvement, and the community, searching for answers, grows suspicious - maybe even hateful. This is not super likely but if some false evidence of the kids' involvement with their parents' deaths is planted (or evidence is misinterpreted) it may be possible.
What if the siblings did accidentally kill their parents, via magic? Unable to reveal their powers, while showing signs of clear involvement with the deaths, these kids will be seen as psychopaths.
No Deaths After All
Perhaps their parents aren't actually dead, unbeknownst to the community - and the kids are just really bad actors. The community perceives the kids as heartless.
Notice This question seems to mark a border between Worldbuilding and Storytelling. Nonetheless when removing the story part we still have a Worldbuilding issue about societal rules and perceptions. Thus it is on-topic.
The answer is quite easy and can be given by referring to a social stigma that's been around since mankind can remember:
Make the siblings Twins
Twins are not normal. They are different and sort of a freak accident of nature proven by their very existence. They are seldom and seem weird and strange to others.
E.g. quoting Wikipedia (emphasis mine):
Many cultures have mythic or folkloric explanations for how twins are conceived. In Greek mythology, some twins were conceived when a woman slept with both a mortal and a god on the same day. One of her offspring thereafter had godlike qualities, and the other was an ordinary mortal, such as Heracles and his twin brother Iphicles. In several Native American cultures women avoided eating twin fruits like double almonds and bananas because it was thought to increase the likelihood of twins.
The excerpt points out a tendency towards perceiving twins as different, wrong, freakish which could hence serve as a reason to wanting to get rid of them.
Story-Note: Your twins being the children of the Founding Family would then counteract this common feeling as long as their parents were alive. Parents gone > no reason to not shun them.
Make the kids neurodivrgent. I speak from experience saying that while even though physical disabilities are still common subjects of hostile ableism, with invisible disabilities an ableist will not be as averted by vanity. There are several more possibilities than the examples that I'm giving, and more to the disabilities than I'm listing, but here's stuff to start out with. Keep in mind is that even people with the same disabilities will not necessarily have the disabilities in the same way. As well, source whatever research you can from people who have those disabilities- they know their experiences better than their mother, or some scientist.
Dyspraxia: This is a neurodivergency characterized by poor motor skills, sensory processing issues, executive dysfunction, and poor short term memory. A lot of this can be misconstrued as laziness. It's easy for a mean person to get mad at a kid for forgetting what they've just heard, or for not being able to handle strong scents, or having illegible handwriting. Other children might sneer at them for stuttering, and the adults dismiss it as kids being kids. In my personal case, the worst part of my dyspraxia was that I was hypersensitive to pressure and movement of my hands;I cannot write painlessly to this day. I had a lot of trouble in gradeschool for not writing; The teachers didn't believe I couldn't write, and made a point of how unreasonable they thought I was. I was treated like I was pretending, even though it really did hurt for me to write. My IEP wasn't followed without begrudging, and that was only when it was followed at all.
Tourettes Syndrome: This one is characterized by involuntary sounds and actions. Your protagonists might twitch their eyebrows, make noises, slap themselves in the face, or say things, all without intending to. This can be interpreted by a hostile observer as attention seeking.
- Sensory Processing Disorder: This is where somebody is either hypersensitive, hyposensitive, or sensory seeking. Sensory processing disorder generally exists as a part of other disabilities, like autism or dyspraxia, but it also happens on its own.
Step 1: The kids need to be different from all the other kids in a way that doesn't scare the other community members, but makes them feel uncomfortable. Like David Bowie style eye colors or an unnatural beauty.
Step 2:Things happen around the kids. The community doesn't know about their magic and maybe the kids haven't (fully) realized it too.
Step 3: Other kids. Other kids can be real (if I'm not allowed to say, please edit out) assholes. They can be role model bullies and start bullying the two siblings. The siblings cause magic by accident, like telecineticaly throwing a knife at one of the bullies when most scared. The bullies run to the adults, adults question the siblings in a rough way, more accidental magic, adults get hurt, things are escalating and siblings are shunned from the community.
You did say they are orphans, right?
Dang, that was a short answer.
But wait, there's more!
The girl can talk to animals. She was always viewed as antisocial due to her habit of wandering into the woods for days on end, and even when she is in town, she doesn't play with the other kids, preferring instead to hold court with her entourage of woodland critters.
The boy is where things get more troublesome. His talents run more to the summoning end of the spectrum: demons. He isn't controlled by demons - much the opposite - but that doesn't mean some of their evil doesn't rub off on him, just a little. No enough to change his actions; but a miasma of evil hangs on him, just out of normal sight, where the townspeople can sense it subliminally and respond to it. Maybe they think he's evil; maybe they notice that other kids around him tend to give in to their darker natures more than other kids; maybe those kids who already have darker natures are drawn to him. Any way, all the townspeople know is that bad things happen around him, whether he's directly involved or not. Now, maybe he can control this leakage once he understands what is happening and gets some training; only time will tell.
Oh, and the siblings' psychic leak doesn't help the issue either.
But mostly, they're orphans.
Merge every reason you can think of:
- Twins that look weirdly similar
- They live in creepy mansion
- Parents were rich and snobby; not even locals but some rich eccentrics from the city that wanted to live in a mansion
- Parents died in freaky accident
- Because of magic weird stuff happens around them: animals behave differently, lights go off and on etc.
=> Lonely orphan kids that no one is looking after => Creepy pale socially extremely awkward creepers
The kids can also do pranks. One of them could go horribly wrong and everyone would know or suspect those kids.
EDIT The prank does not even have to be bad, but something can happen. If a pregnant woman would end up as the victim of a prank and then gets a miscarriage, it would be extremely bad if it were to be linked to that prank. Add a suicide due to miscarriage and the mood of a small community is absolutely wrecked, which for there is those to kids to blame...