(The kids just think their human side au)
What if the kids never knew their parents. Because stan does not know Bill and talking about Ford hurt Stan to much.
So the encounter with Bill happens and instantly Bill knew they were his, but the kids didn't/ don't believe anything the triangle is saying. ( They saw the journals they know better)
And Ford reveal, the portal unleashed energy that causes powers to be revealed and or to destabilize their human forms. Everyone freaks out.
Oh that's an interesting take, like the twins turned into human forms when they were much younger, 1 day Stan has a tiny little Pine tree and shooting star demons the next 2 human babies.
Stan is like '...ok' and just rolls with it, this means he can take kids outside he thinks for a moment they may be human human... until one of the kids floats. As they get a bit older start using the small powers less, Stan doesn't bring it up just treats them like normal kids and they seem like normal kids. Apart form occasional weirdness but Stan just brushes it off and makes sure no one sees he'll do anything to keep them safe.
He 'home schools' them for a while he makes excuses to them not telling them that truthfully he's terrified of them being away all day accidentally using powers and people seeing and some government agency coming and taking them away.
He always tells them he's their Uncle, when they ask about their dad, he says 'He's gone' the kids find newspaper about death of Stanley pines and automatically think oh our dads dead. Stan doesn't go into it he can't it hurts to mcuh and how does he even begin to explain to the kids he's trying to keep them away from the weird stuff he wants them to be human and live a normal life... he doesn't want them at risk he cant bare the thought of losing anyone else.
When they ask about other parent Stan just says he doesn't know, he never met them.
In this AU it's much closer to canon Stan's treated them as human and managed to keep them away from mcuh of the standard gravity falls weirdness, and the kids think he doesn't believe so when Dipper finds journal they don't tell him.
They never notice when they use powers at all, or how things get weird when emotions get strong and they think shared dreams is just a twin thing.
When Bill sees them and senses their powers he knows, he knows they're like him and he see Ford ion them and he's confused and suddenly looking at Stan's memories is for this, he finds out and he is pissed at Stan. He tries to talk to kids but of course thanks to journal don't trust him. Bill retreats back and starts making plans... he now has another goal other than weirdmageddon he's getting his family back and making sure his kids know they're not just human they are so much more than that.
The times when Bill does contact them he;s trying to bait them into using powers, he wants them to know they're his, like him...and he's very sure having such a secret kept form them will turn them to his side.
The portal knocking them back both kids instinctively using their powers and suddenly Fords back and there are a floating pine tree and shooting star... they all start screaming/picnicking.
Stan is contemplating faking a heart attack to get out of this.