Kommentar zu Divrej Hajamim I 18:17

over the Cherethites and the Pelethites The meaning is that they were nations, as it is written, (Zeph. 2:5): “Woe to... the nation of Cherethites,” and from their nations were mighty warriors with David, and Benaiah was over them. Another instance of this (II Sam. 15:18): “... and all his servants passed beside him, and all the Cherethites and all the Pelethites and all the Gittites.”

and the sons of David were first at the king’s hand The meaning is that David’s sons were always first beside the king to perform all his necessities.