just last week, we covered Jeremiah 7 in class where the prophet accused his contemporaries of offering child sacrifices at Topheth in the Valley of Hinnom. the Valley of Hinnom at the southern end of the city of Jerusalem was used to burn rubbish and by the time of the NT, it was obviously still in use. Jesus referred to the valley as Gehanna because of the constant fires.
but how widespread was child sacrifice in the OT? the author, Heath Dewrell, who lectures in Princeton, argues that it was not widespread but the practice was present among some of those who were Yahwists i.e. those who worshiped Yahweh.
some of the people had assimilated pagan worship to the god Molech or Moloch with Yahwism. hence, they were following the practice of offering their first-born. but at the end of the day, thankfully, those who advocate banning such practice won the day and the practice stopped.