Because no one is born toxic, it is created within us. For example, in my case, I can trace the toxicity all the way back to my great-grandmother (it probably goes further, but that is the furthest back I can pinpoint without a doubt). My great-grandmother favored her sons over her daughters and her attitudes and behaviors exhibited as much. My grandmother was the same way. My mother repeated this. The cycle was destined to be repeated with my lot, but fortunately, I recognized it and have now changed my ways.
My grandmother, my mother, and I were once children too; none of us were toxic then but because of the behaviors we had learned from our parents, we became toxic adults who chose to become parents.
Don't misunderstand me, adult children can be toxic (I was, once upon a time, living proof) but as I want people to understand, it is (99% of the time) learned behavior, just as it was for at least three generations of my family. For the toxicity to end in a family line, someone has to be brave enough to do some serious inner-reflecting, confront the issues, and find a resolution even if it means walking away from the toxicity if the source(s) of these learned behaviors is unwilling to work on healing themselves.
This isn't an issue of parents vs. children. This is a matter of healthy minds vs. disordered minds. If you are willing to look at it from this perspective, it can have wonderful results in your life.