Your question about the Key for Harry Chapin's Cats in the cradle is classic and has been discussed before.

"...The A, C, and D are based around fitting the A minor pentatonic melody. Basically blues let's you get away with mashing any major chord into that scale. The walk down from G is a temporary modulation to G major, which as a key isn't very far from A mixolydian, which I'd put the overall song in. Now in the G major section normally the walk down would take you from C to Em, then Am, D, G. But at this point he's strongly established A major as the home chord. If the song were in G, the A major would sound like a secondary dominant pulling to D7, but it doesn't. Your brain lets that short section in G jump right back to A." https://www.reddit.com/r/musictheory/comments/65t0ge/why_does_cats_in_the_cradle_work/

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