The accused, who had been arrested, took the girl from her mother on the pretext of taking her to a park in Uttam Nagar area on Thursday evening and allegedly criminally assaulted her, police sources said on Friday.
When he brought back the infant, she was crying uncontrollably. Her mother then found injury marks on her private parts and approached police.
The boy was later arrested and sent to a judicial remand home, the sources said.
The accused is a fourth standard student in a public school and his father is an employee of Municipal Corporation of Delhi.
Further investigations were on into the case, the sources said.