I taught young kids for a while.....I gave them music just start with simple tunes using only a few notes.. they soon learn how to read them. (I start with a simplified "Mary had a Little Lamb" 3 notes B,A, & G... then move onto more notes slowly with things like "Twinkle Twinkle little star")
we've had the discussion on types of whistles before... personally I don't like Susatos.. can't trust them in the top octave.
For kids..... good old cheapy Generations.... (and I start them all on D's), or Sweetones (pretty colours, and marketted as being gentler for parents/ teachers to listen too)....
keep it interesting too.. use tunes they know..... my lot thought it was fantastic when I taught them the whistle bit from "My heart will go on" (Titanic).
If you want me to send you the GIFs I use of simple tunes..let me know...
Have fun...
slainte
alison