Well 'impossible'? We're talking comics here.
Well, yes, but the fact that the Vision can't actually father children is kind of an important point to the plot in the comics....
And if it is set in a mental construct, no reason they can't create kids mentally.
In the comics, the kids are entirely real.
Maybe you folks don't realize the scope of Scarlet Witch's powers in the comics. She doesn't just play a few mind games and blast red energy around. She's reality-altering.
She conceives the kids via magic, but that goes awry as she seems to have taken the soul-energy necessary from a bad guy (Master Pandemonium). Other magical heroes return the soul energy, and wipe Scarlet Witch's mind of the whole debacle. But mind-wipes are never reliable...
One of the Avengers (Wasp) actually mocks her ability to have children. This brings back memories, and she's seriously cheesed off. With a bit of extra-planar help, she launches a campaign of terror against the Avengers for what has happened to her, killing several Avengers....
Then, realizing that it looks like the Avengers and X-Men have decided to just try to kill her, her brother (Quicksilver) convinces her to create a whole new reality, in which everyone is just happy (the "House of M" if you care to look into the comics). Magneto eventually figures out what was going on, and kills Quicksilver for it. Now, completely enraged, Scarlet Witch ressurects her brother... and then de-powers 90% of the mutant population as a bunch of meddling jackasses.
And, somewhere in all this, her children are reincarnated/resurrected. They are the current heroes Wiccan and Speed.