Most lesson books have, at some early point, both thumbs playing middle C in turn, while finger 5 of each hand is over treble G and bass F.
My first piano book's first song was exceedingly simple*:
LH: C C C C C C C C
That's middle C, of course, with all quarter notes in 4/4, fingered 1-1-1-1 etc. The stirring lyrics:
"Left, right, left, right, I am singing.
Left, right, left, right, while I'm playing."
Pretty exciting stuff for a four-year old!
*(Sorry about the underscores, that was the only way I could find to make the forum respect my spacing.)