Cause one … to stumble: This means to lead believers to turn away from Christ, whether children or adults who have become “like children” in faith (see Matthew 18:3).
The title little ones emphasizes their humility (see 5:3–12; 18:4; 19:13–15).
The thought of being drowned with a large millstone around the neck is terrifying. But Jesus warned that causing someone to lose faith will bring even greater suffering (18:7; 26:24; Revelation 18:21; Matthew 22:13).