It's really daunting to call time on a relationship whatever age your children are, but personally I think that if you get to the point where you feel the things you've written in your OP, that it is actually the right thing to do, and things will not get any better if you stay and stick it out instead. "Crap not terrible" is a very familiar concept to me and I remember well how it instinctively makes you want to reach for the handbrake before you hurtle off into decisions there's no coming back from. But the idea that things ought to actually be "terrible" to justify all the upheaval of leaving comes out of the playbook that says your marriage is "good enough" if no one's hitting anyone and the housekeeping isn't all being chucked away in the pub. This is the 21st century, and it's okay to walk away from relationships that don't quite plumb those depths but nevertheless are crap and are creating so much unhappiness it feels unbearable, as you wrote.
I left a 25-year relationship earlier this year and right up to the point the house was sold and things were final, I was still second-guessing myself and scared I was doing the wrong thing or going about it in the wrong way. But once you have the inner peace of being in control of your immediate environment, the decisions and choices you can make, your parenting and your plans for the future, the relief will wash over you and you will wonder why it took you so long. Counselling is a good move imo, as people in unhappy marriages tend to suppress their feelings for so long it can be hard to access them even once you're free to.
Lots of good advice upthread about practicalities, and I echo the warning not to assume that your stbx will behave well. No need to behave badly yourself preemptively, but just be ready to act decisively when ringfencing assets etc. In my experience (looking at other relationships as well as my own) there's often a window of opportunity when everyone is trying to be/look as reasonable as possible, and my advice is to take all you are offered in that window, before things sour. (They may well become amicable again later, but the sour period can do a lot of damage, especially to your finances.) As for telling people, I would limit it at first to people who will be supportive (of you). Ending a relationship is energy sapping and you'll need friends or family who can bolster your confidence and belief that you're doing the right thing at times, as well as people who you can just have a laugh with when you need one. You don't need to rush to tell people who will judge or try to make you doubt your judgement - trust me, they'll rush to you - and you don't need to offer yourself up on a plate as anyone's juicy gossip (that will happen by itself too). Finally, be aware that there are a lot of people out there in unhappy marriages, and your decision will make some of them uncomfortable about the decisions they're not brave enough to take in their own lives, so you may lose some friends or take some flak.
You will be okay though, more than okay, and so will your children.