Currently it’s being reported that WandaVision are looking for twins that are both approximately 8 year old. The characters are being code named: Timmy and Adam. Marvel comic fans know that these are just pseudo names for their actual names: Wiccan and Speed.
Rumours also say that the show is going to be based in a few different eras, hence why they may be grown already and they may even become teenagers throughout the show. It was previously noted that the show was seeking young babies for the show, however nothing is confirmed and guaranteed.
Wanda and Vision have previously had a family in the comics, but it seems that after the events of Endgame, the shows are taking different approaches and using completely unique show plots and ideas.
WandaVision will allegedly also have an audience for some episodes, maybe a sitcom?
No one knows how exactly they’re going to have kids, considering that Vision is a robot and Wanda is a living organism/witch, however in Age Of Ultron, it was stated that the Vibranium in Vision’s body was ‘bonding’ with the cells. Maybe he does have a penis?