According to my dad they were simply herded into a farmhouse. It was like that all over Poland. I am not a hundred percent sure but I think they were basically imprisoned in their own homes. The Germans shut down the borders. Or, maybe my grandmother didn't have her U.S. citizenship yet. My dad and his brother were born in the U.S. but my grandmother was from Poland. I never did get the whole story. My dad passed this past May so I guess we'll never know.
That probably explains it.
Not my area, but it's my recollection that they would have been considered to have dual citizenship since they were born of Polish parents. The mother probably did have a window in which she could have appealed to American diplomats to help get the children out, but she probably would have had to travel to Berlin (or at least a city with counsulor offices) to make the appeal, and probably couldn't do so.