This is the start of a rhetorical question. Jesus used it to introduce a comparison that he was about to make. This whole question could also be translated as, "This is what I will compare this generation to. This is what they are like." (See: [[rc://en/ta/man/translate/figs-rquestion]])
This simile is the beginning of Jesus' comparison. Jesus is saying that the people of this generation were like children who were never satisfied with the way other children acted. (See: [[rc://en/ta/man/translate/figs-simile]])