timdiggerm said:

It's hard to say, from a 6-movie perspective, what Owen and Beru know or don't know. They know that Anakin left suddenly, immediately after his mother's funeral, following Obi-Wan on some damn fool idealistic crusade. Obi-Wan must have given them some sort of explanation for who he was and why he was giving them this baby.

I always thought that there might have been a bit of dialogue there that was cut out. Beru does turn and walk away pretty suddenly directly from a cut. Having dialogue there though would have broken the mold of all the movies ending with a few minutes of stuff happening on screen with no dialogue as the music swells.