I've never had this event myself, even after 440 hours plus of gameplay, but from what I've gathered, the Demon spawn will kill off everyone in the way of their claim to the throne, which isn't an issue in your case since they're already heir. They apparently end up with an insane level of attributes and great traits, so I'd say stick with it, and enjoy the ride.

The demon spawn character will have great stats and should get the genius trait when he hits adulthood. It is a rare event so enjoy it.

its time to go OP man, just let it happen and conquer the world.

I already have most of the world conquered, probably gonna finish off Germany, France, and India with the demon guy.

Lucky! Don't worry about the consequences: dead pretenders, he kills you (possible, has not happened to me), lowered temple vassal opinion, lowered piety per month and some one-way switch to heresy events when you play him

They're a beast, just make sure to give them titles to make them heir, otherwise they'll kill everyone above them in line of succession except you. Or use them as a convient way of getting rid of unwanted children.

The demonspawn though will get like 20-30 stats in every catagory once they hit adulthood, as well as an army with three decently stat-ed Witch Generals you can put on your council.

It is a fun event and you should play trough it once atleast :)
Lets hope it doesn´t bug out. cause it really goes trough a lot of ages till his death.
But your dynasty comes out way stronger even if it feels it isn´t like that at start.

And probaly you will get Joan D´arc event afterwards, those two made a huge impact on how much I enjoyed the CK2 events. the republic one sadly wasn´t as good :(
Still haven´t gotten the horselord one, ah maybe with the next DLC testing phase. when they release it.

with elective the demon child may kill vassals as well

yay literally no events happened when he ascended to the throne and i had to convert away from heresy to avoid getting indepedence warred on, great event

Demon child is fun to do once, but otherwise it wipes out family members and can put your whole dynasty in danger. Especially if it kills you off and you're subject to the arbitrary "Can he survive to 16" nonsense, despite being satan or ahriman or whatever. You can shoot fire out of your ♥♥♥ and kill with a gaze, but some ♥♥♥♥♥♥♥♥ can still push you off a castle wall to your death.

eh the events never fired anyway, some guy dueled and killed him. not all bad though, now the Genius trait is within my dynasty's bloodline

Traits don't work that way. It is generation by generation. If no child of a genius parent has genius, it won't show up in future generations unless reintroduced by marriage. The genetic traits can only be inherited from a direct parent, not from grandparents and beyond. There is a flat 15% chance of inheriting genius from a genius parent (15% from each if both parents have it). Some traits have a higher percentage, like dwarf (50%), but it isn't cumulative. The game checks the parent with the highest title, a 50% chance, then the other parent for another 50% chance. So it is possible for a child of two dwarf parents to not have dwarf. And that child's children will not have it unless another dwarf is their other parent. Works the same for all of the genetic traits.

http://www.ckiiwiki.com/Breeding#List_of_Congenital_traits