The ad featured a snapshot of a little boy, dressed in blue, with a caption reading: "Cries a lot and did not let me sleep and I have to work to survive."
Police in the central state of Goias have launched an investigation into the advertisement, which was posted on Tuesday on the OLX classified website.
Child protection officer Marcela Orcai said they were trying to locate the person who placed the advertisement, 'New York Daily News' reported.
According to 'Jornal do Brasil', OLX has denied any involvement between advertisers and customers using its platform.
However, the advertisement was removed from the website for violating its terms and conditions.