ABC is a well-supported method for expressing musical notation in text, i.e. with note letter-names.
Using ABC notation, and in the key of C, Mary Had a Little Lamb might go: ED CD EE E .
If, like Sister Maria, you use
solfege,
which can be used in "key-agnostic" ("movable Do") mode, then the song would go: Mi Re Do Re Mi Mi Mi .
The present 123 notation is also key-agnostic, and gets parsed into ('boils down to') ABC.
In 123, the song would go: 32 12 33 3.
This 123 site lets you enter / edit 123, and you can either use the default key,
or you can specify a key from several choices.
The songs included under the Sample Songs tab range from simple use of 123 to more-complex use.
Try using the Load Sample dropdown to select and then play the one called 'kids'.
The sample called 'demo' illustrates many of the features of the 123 syntax.
You can see (or edit) both the 123 notation and the ABC generated-code under the Compose / Edit tab.
Or, to check out one of my original goals on this site, try loading the 'twelve' sample song.
Then, under the Rainbow tab, display the Rainbow Grid, and click 'Infinite Play and Shuffle'.
(NB: Not-so-simple constructs, such as triplets or chords, will play better
in the Sheet Music tab than in the Rainbow tab.)