We don't know and it doesn't matter
Your question as stated gets most of the way to the answer, as the two children in WandaVision are creations of Wanda's reality-altering magic, as is the Vision in that series. As such the WandaVision are not real 'children' have no 'father'.
What we learn in Dr Strange in the Multiverse of Madness is that Wanda's dreams of the children are actually reflections of real children of other variants of Wanda in other universes. The children of WandaVision are therefore the product of that subsconscious knowledge.
The variant of Wanda in the 838 universe actually really lives in WestView with her two real children. Their father never appears in the movie, is never mentioned, and provides no childcare when Wanda leaves them unattended having been possessed by the dreamwalking Scarlet Witch/Wanda from our universe.
It seems highly likely that these children had a real father, but he is away, travelling, or no longer on the scene. And from the point of view of the overall story, it just doesn't matter.