It is a sad situation, it would be nice to allow them to be together.
The problem I see is that once it's allowed there would be thousands of hard luck stories and requests and that would create a whole new bureaucracy just to check out the truthfulness and merit of the stories.
The well being of a dying child deserves it own new bureaucracy. God knows we have enough money to waste on everything else. I’m sure there are enough charitable organizations that would be willing to donate for such a cause.