It has long been apparent that the twin sons of Wanda and Vision would be making an appearance in this series, first hinted at by the presence of two cribs in the "Big Game" spot for the Disney+ shows.
It was also indicated by a leaked test screening for a child actor asking about an "uncle," which is likely Wanda's brother, Pietro, who died in Avengers: Age of Ultron. So, fans knew that they would see these children grow up over the course of the series at a likely rapid rate.
It is incredibly apparent, especially with the official announcements of Ironheart and America Chavez joining the Doctor Strange sequel, Marvel Studios is pulling all the stops to have a team of much younger heroes in the universe. This is likely as a means of forming the Young Avengers or Champions to have their own spin-off show or movie. Despite all of this, fans still haven't gotten a look at the actors who will be playing the twins.
A photo of at least one of the twins can be seen briefly in the background of the newest WandaVision trailer, which takes place in the modern-day era of sitcoms, as seen by their clothing and the furniture.
WHAT THIS MEANS
It is all but confirmed at this point that audiences will see Wanda's twin sons from the comics, Billy and Tommy, in WandaVision. It's impossible to tell who Wanda is holding in this frame, but it's likely an entire family portrait being blocked by Vision. Additionally, earlier in the trailer, viewers see a dog run out of their home, likely Sparky from Tom King's Vision. Upon closer inspection, fans can see at least three others following the dog outside, again, at least one of them possibly being one of the teenage twins.
Not to mention, once again, a crib is shown, further indicating that Wanda and Vision will have children in this show, as all sitcom families typically have to complete their classic nuclear family structure. This new trailer makes it pretty clear that there's someone else manipulating Wanda or interfering in her newly created reality. It could, in fact, be Mephisto like in the comics, who wants Wanda's children for himself. After all, it seems that a visual reminiscent of the demonic being was seen in the trailer for Loki.