This performance by John Sebastian didn't make it into the movie Woodstock, although this video suggests it was edited for it. It's one of my favorite songs by The Lovin' Spoonful.
It's the song of the expectant parent, and an idea and concern you don't hear expressed that often. Certainly if I were, I'd do it somewhat less hippy-dippy than this... I'm sure that the Sebastian of today would, too.
But it's hard to beat "'cos I swore when I was small that I'd remember when I knew what's wrong with them that I was smaller then." as a sentiment that's hard to avoid thinking about as the days go on.