Who is the little boy that Moocow meets in James Joyce's "A Portrait of the Artist as a Young Man"? Is the Moocow Stephen?

Yes - Stephen is the moocow. In the first section of the
first chapter, we get the first glimpse of Stephen's observation. The narrator says "he
was baby tuckoo" meaning that his father always told him that he was baby tuckoo. This
is a very basic illustration of what Stephen remembers as a little boy. Think of your
first memories and how disjointed they are and you can kind of get the drift. At the end
of the section on the second page of the novel, Stephen is hiding under the table
because he apparently told his neighbor Eileen (who is Protestant - Stephen is Catholic)
that he would marry her when he grew up. As such, he is told that he will apologize or
else an eagle will come and pull out his eyes. These are all first memories of
Stephen's.