It should be “lest we forget”

I know you only sometimes think this (or think that sometimes people have kids for the reason you state), but I can chip in a data point for myself where I have never been sad about how my life has changed as an adult, my kids are very happy, and I’m very happy for them! I am sometimes magically reminded of my youth because of them but I do not seek to feel the same again. I know why I had kids and none of the reasons were to personally relive my youth vicariously, but I acknowledged that they would likely feel many of the same great feelings I did as a kid (and good for them!).