Maybe it's different in the US, but in the UK there's no statistical difference between earnings 3.5 years after university graduation due to parental background once you adjust for university/degree subject.
(children from poorer backgrounds are less likely to go into good universities or study high-earning subjects, but if they do then they'll earn as much as their better off peers)
That's quite an interesting observation I don't seem to have seen trumpeted (enough) during the battles over tuition fee increases. Do you have a citation for it?
In some areas (e.g. law) I wouldn't expect there to be much difference in that time - in Scotland at least, 3.5 years after you graduate with a law degree you've only been qualified for 6 months! (The law degree is followed by a postgraduate course for a year then two years training before you qualify).
I'd be interested to see the results for 10 years after graduating!
Only a tiny percentage of law graduates go onto become lawyers (<10%), so it's probably doesn't have a significant impact on the overall data even for law degrees.
Certainly it's possible that in some careers that there is a divide, but due to the the careers not being common that it doesn't show up in the stats. But there's no evidence this is the case.
(children from poorer backgrounds are less likely to go into good universities or study high-earning subjects, but if they do then they'll earn as much as their better off peers)