To me, the last line reflects a really terrible view of human nature on the part of Robert Frost (or at least on the part of whoever is narrating the poem).
The last line calls into question the relationships between the various members of the family. It implies that they didn't really care a whole lot about their son. Once they figure out that he is dead they go about their business because they are not the ones who died.
So, overall, this line turns the whole feel of the poem around. From the time that the boy has his accident until the last line, you assume that everyone feels awful. But then it appears they don't care.
This makes the whole poem seem cynical and like it's saying that human beings don't really care about each other.