Jesus was not David's literal son, so this may be translated as "descendant of king David." However, "Son of David" is also a title for the Messiah, and the children were probably calling Jesus by this title. See how you translated this in [Matthew 21:09](./09.md).
It is implied that they were angry because they did not believe Jesus was the Christ and they did not want other people praising him. AT: "they became very angry because people were praising him" (See: [[rc://en/ta/man/translate/figs-explicit]])
The chief priests and scribes ask this question to rebuke Jesus because they are angry with him. AT: "You should not allow them to say these things about you!" (See: [[rc://en/ta/man/translate/figs-rquestion]])
Jesus asks this question to remind the chief priests and scribes of what they have studied in the scriptures. AT: "Yes, I hear them, but you should remember what you read in the scriptures ... praise.'" (See: [[rc://en/ta/man/translate/figs-rquestion]])
The phrase "out of the mouths" refers to speaking. AT: "You caused little children and nursing infants to prepare to give praise to God" (See: [[rc://en/ta/man/translate/figs-metonymy]])