Keep her as your heir and have !!FUN!!

As soon as possible, marry her off to someone as far away as possible, and make in their problem! There must be somebody, somewhere who'll take her off your hands. The higher your title and prestige, the better your chances.

If you have Way of Life DLC, you could try focusing on "intrigue" so you can spy on her, leading to various opportunities to kidnap her into your dungeon, or uncover reasons to imprison her. Either of those avoids the high "tyranny" opinion penalty of outright imprisonment without a reason.

Alternately, if you only have a few vassals, you could consider a "reign of terror" where you imprison everyone including her, revoke everyone's titles, and then install a whole new set of vassals and councillors. The tyranny penalty won't apply to them, because they were installed AFTER the tyrannical events.

However, demon children have a reputation for escaping dungeons, even the oubliette. Marrying them off is probably safer.

If you don't have agnatic-only succession, just roll with it.

Yeah I have to agree here if you are lucky enough to have a demon spawn just go with it and enjoy the experience because who knows when or if it will happen again. Let her kill, ♥♥♥♥ help her out by giving her targets that could be beneficial to you to be rid of in your own dynasty. Let the little evil satan child have some fun and see where it takes you.

I'm so looking forward to the next DLC. Immortal demon child leading a cult of satanists towards world domination. Start in the Chaos Wastes Perm for extra points! Awesome!

got this problem; my daughter killed my 3 sons but she failed to kill my last who get strong trait (he survived during her night attack) and became my heir.
Then i married her because i was playing Zoroastre religion! genius trait + great stat wife

I've got to agree with the "Roll with it and have fun". The one time I've managed to deal with a demon child without it becoming my heir was when he was born to one of my cousins. I killed of his father as quickly as I could, and married off his mother to some poor noble on the other side of the map (making him somebody elses problem). I was in England and Damien ended up in Italy. He eventually led the Cathars in seizing Rome and installed himself as the Pope... Good times

Recognize the event chain and smuther the child when the option arises.

I liked the chain the first time... once is enough. You get a powerful ruler that's good for breeding, but terrible for educating.

About 30 years into my Sunni game I have a Demon daughter (3rd child). Never had the event before so decided to roll with it - after all what can go wrong - right?

Anyway, about a year after her arrival my ruler dies, with eldest and only son (age 6) inheriting.

After that I was screwed. Demon child killed my infant ruler - instant game over as no other heir.

Restarted 5 times, rolling back several years (unfortunately no save from before my adult ruler died) and each time she assassinated him and the other siblings. Each time its game over. No apparent way to eliminate her as I cannot jail my own sibling while still under a Regency :-(