Wall or no wall people are going to come to America as long as that looks like a more appealing choice than staying wherever they were at first. Those kids died because they weren't treated well when they came up here for whatever ailments they had (Sorry but I'm not an expert on the details, its just super fucked up they died at all)
It's more subpar conditions that they're being kept in. While media and others are not allowed in these facilities, migrants detail conditions of squalor: over-crowding, undernourishment, ignoring illnesses, and vomiting & diarrhea being common place (including said messes being left alone for hours or days). Under these conditions, airborne viruses would spread like wildfire, and indeed one of the children, Felipe Alonzo, died of influenza B.
Restated, a good way to kill a bunch of people is to pack them into a prison like sardines in a can, and just ignore whatever happens. Nature will kill them for you, you don't have to deal with the negative PR of racism or genocide that would inevitably occur from more active measures. All the while, you get what you wanted the entire time. Stephen Miller, you sneaky sneaky goose.