Wisdom of our forebears

So I appreciate the real-time parenting advice of my peers a lot. Time moves at warp speed and memory is fickle, so I don't usually count on the how-to of the parents or in-laws to get me through the day. Plus, the times they are a changing and the 'rents don't always understand what we're going through (Debates on whether or not to get vaccinated? Why in the world?)

However, I feel like I'm getting no input from these seasoned veterans. I need some big picture advice.

I wish I could poll a katrillion grandmas and grandpas (and great-grandmas and great-grandpas) and ask them what the one thing is that they wish they could change about raising their kid. And then the one thing they are so glad they did do. I'm guessing some really good advice would rise to the surface.