Worldwide, child labor is most prevalent in the agricultural sector, where children are widely used as day laborers. Working long hours, children working in agriculture are often exposed to harmful pesticides, dangerous machinery, backbreaking loads, and many other hazardous conditions.
Working in agriculture is not good for a child's welfare.
Children should not be working as laborers anyway, but the article points out several aspects of agriculture which are particularly dangerous (pesticides, long hours, machinery) which are obvious threats to a child's wellbeing. I think the system has difficulty when you paraphrase things instead of using words from the article. 
( z1 good :polarity - :ARG1 ( z2 work :ARG1 ( z3 agriculture ) ) :ARG2 ( z4 welfare :poss ( z5 child ) ) )