The family sits down for a nice brunch. The boys sit on either side of their mother.
Less than a minute later, they've vanished.
I'm guessing one of two things. Either:
1. The boys, working under child film laws, could no longer work for the day but the shoot had to go on. Or...
2. Originally, the script contained a moment where the boys were ushered away--to save time, money, and a potential problem with child labor laws above--and that moment was edited out of the scene.
This kind of thing happens frequently. And, honestly, the audience doesn't care, assuming they notice in the first place. But such situations should serve as a reminder: Always keep your scene as simple as possible. Do we need the children? Probably not. We could have had two less bodies on camera, helping speed the process along, and keeping the scene focused.
On the other hand, the three seconds we see of the boys does help sell the idea of "family" before they magically vanish. So, perhaps it was worth it in this case.
Just keep this in mind: If they can be cut in the editing bay, do you really want to take the time and effort to put them in scene in the first place? For those of us working on zero-budget productions, the answer should probably be: No.
Your Media Production Mentor