Probably because normally there's a huge financial and social disparity between shoe shiners and the people getting their shoes done. Classism is a learned behaviour so it's disconcerting and unpleasant seeing such young children who wouldn't yet understand classism behaving in discriminatory or prejudiced/internalised oppression behaviours.
Additionally the shoes shining child has dark hair and is wearing what appears to be a fez/tarboush whereas the child getting their shoes shined is blonde - which brings to mind social disparaties between white people and POC/colonialism and could be seen as enforcing racist stereotypes, again in young children who wouldn't normally have any sense of this sort discrimination.
Tl;DR: It provokes a juxtaposition of innocent children against adult behaviours that involve classism, racism and colonialism.
It's like the Uncanny Valley of classis, racism and colonialism - it illicits feelings of discomfort but you can't entirely put your finger on why.