the queens niece has two sons called arthur and samuel and i think they go well together. my cousin arthur has two younger siblings called edward and eleanor. Names i think go with arthur are
Arthur and Charlie/Charles
Arthur and Jem/Jeremiah
Arthur and Albert
Arthur and Sam/Samuel
Arthur and Jacob
Arthur and Freddie
Arthur and Oliver
Arthur and Sebastian
Arthur and Rose
Arthur and Liesel
Arthur and Anne/Annie
Arthur and Molly
Arthur and Beatrice/Bess
Arthur and Josephine
Arthur and Violet/Viola
Arthur and Sophie
Arthur and Isla
If you like the regal feeling of the name Arthur, names like Charles, Albert, George, Henry, Edward, James, William, Louis and Alfred for boys and Alexandra, Mary, Elizabeth, Alice, Victoria, Beatrice, Eleanor, Louise and Charlotte for girls might go well.
If you like the traditional name Arthur and go for the nickname Arty, another traditional regal name, Alexander has a creative spin with the nickname, Sandy too. Ella and Lottie, short for Eleanor and Charlotte too etc.