[harmegido]

I would guess that the author read the study from which the report is based, and noted the section defining 'Child Income' :

"Our primary measure of child income is total pre-tax individual earnings. For single filers, individual earnings is defined as the sum of wage earnings and net self-employment income"

They go on to say how they also measured child income in the same manner as for parents (including investment income, etc) to see if that made a difference.