So if I understand correctly it is no longer possible to change culture/faith of your children by any means?
That is absolutely not cool, I liked to change my catholic croatia to muslim if I had the chance to, it gave new playstyles and made the game interesting. I also liked being able to switch from catholic to orthodox as I like the orthodox playstyle more. This is not cool.
No, what's been changed is that if you, say, have a random Sunni in your court as a Catholic king, then having that Sunni educate your children won't make them Muslim or Arabic at all.
However, if you send your kid to be educated by a courtier in a Sunni vassal's court, then the kid can change to Sunni because the employer of their guardian is Sunni. I think from what the patchnotes imply, it'll also be more likely for the AI in different realms to accept guardianship of your children.
If you wanted to convert a child of yours to Judaism, for example, then you could just give a Jewish courtier land, and then have them or one of their courtiers educate your children. Thus, they could become Jewish. At least, that's what I picked up from what the devs have been saying.