It is literally possible to cull a human being. It would be a morally reprehensible thing to do, but it is certainly possible.
When you "cull" an individual, it means you are removing them from a group to improve the quality of that group. In human beings, this would mean doing something like killing people you thought were in some way substandard. Today we would not think about doing this, but it has been done in the past. The most notable example of this would be the Nazi killing of mentally retarded people in the years before WWII. (Even the Holocaust can be seen in this way.)
Without knowing what is going on in the passage you quote, I do not know exactly how or why the parents are "culling" this child. However, it is possible to do that to a human being. It just sounds odd because we would not do that today and therefore we don't speak of it.