Nozick disavowed the argument DeLong explains so maybe it unfair to say it is his.
Here is DeLong's takedown. I figured that out when I was 16 so Im glad to get validation 5 years later.
Also, although it is a good point, I think the main point to make to libertarians isn't that they are (apparently) willing to treat children like shit. The main problem is that they believe in perfect markets. I think without that faith they'd be less willing to shit on children.