Alvin and Millie are apparently both dogface children and friends of Morty's. These tales take place during the late 1950s through the early 1970s when Ferdie had been dropped from the "Mickey Mouse" strip by Gottfredson, and Morty was then living twinless with his Uncle Mickey Mouse. Alvin first appeared in the Mickey Mouse daily strip on January 26, 1957. Eight years later, Millie was introduced to the Mickey Mouse daily strip on February 3, 1965.
Alvin and Millie are NOT siblings, since in one strip Morty calls Alvin's house and speaks to his mother, then calls Millie's house and speaks to her mother (Mickey Mouse daily strip; March 17, 1968). Alvin appears in several strips to be Morty's best friend, while Millie may be Morty's girlfriend since she and Morty double date with Mickey and Minnie Mouse (Mickey Mouse daily strip; February 7, 1969), and Millie spent a good amount of time at Minnie's house without Morty (Mickey Mouse daily strip; November 27, 1966). There doesn't appear to be any connection between this Millie and a "Millicent" that is supposedly one of Minnie's Nieces in the European marketplace. These two children were regulars in the King Features Syndicate Mickey Mouse Daily Gags which appeared in newspapers during the late 1950s, and shortly began appearing in the Sunday funnies as well. Alvin was consistently used in the mid-60s, and his peak year was 1965. He appeared in at least 50 dailies. Note: there were two dailies in 1963 (9/25 and 12/6) where a little boy called Alvin appeared, but he was not the usual chubby, blonde-haired kid; in the former daily he was thin, whereas in the latter he was dark-haired! It is possible that "Alvin" was used as a general name for Morty's friends. Millie, too, was consistently used in the MM dailies and debuted in the Sunday strips July 4, 1965. She was still appearing in 1975 when Gottfredson quit writing the strips. The comic strips where these two characters appeared were drawn by Floyd Gottfredson and written by Bill Walsh, Roy Williams and Del Connell. Most recently these two appeared in reprinted strips in a book called "Mickey Mouse Fifty Happy Years" (The Official Birthday Book), edited by David Bain and Bruce Harris, and published in 1977 by Harmony Books, New York.