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]])