I wish it was more socially acceptable to not have children. In many parts of the world, there is still so much stigma if you marry and don't have a child within a couple of years. I am hopeful things will change and we can swing down the total fertility rate to below one.
I'm 34 and living in Berlin - I know a low of people within ±10 years of my age here and MANY have no kids and aren't planning on having any. The norms are changing in a lot of major urban areas.
If I "ambitiously worked on side projects", I'd at best miss times I'll never get back and at worse be downright neglectful.
That's a "trade off" I haven't thought twice about making. There is no mistake. As I said, my kids are number one.
In fact, I'd argue it isn't much of a trade off at all. Time with my kids is infinitely more valuable than time spent on my career.
Someday :)