It's more important to understand chord progressions and relationships than scales, and that's a lot of the philosophy of Suzuki and related approaches— you learn to sound out a melody, and then you learn what major chords sound good with that melody, and then you learn where to substitute an appropriate minor chord, and then you learn how create an accompaniment using those chords.

And at that point you've learned to recreate a song you already know, and it's not a huge step to improvise from scratch on your own melody.

There's a huge body of potential source material for this approach, including folk/pop songs, oldies, scores from musicals (think Disney Renaissance era), and most contemporary religious music.