Maybe Homer or Marge painted those pictures and they just happened to look exactly like their future children!
The Simpsons staff has already committed to the fact that this is a blunder, although there is a chance the writers can still use this blunder to their advantage for a future episode plot or even a movie.
We already know the show is really good at predicting future events, so perhaps there’s something weird happening in Springfield that has made its citizens subconsciously clairvoyant?
It’s a crazy premise and it’s unlikely the staff would run with such a concept outside of a “Treehouse of Horror” episode, but wouldn’t it be great to see a canon explanation of how these pictures are present in the past?