I'm another person who could have written your posts over the last few years.
Debating whether I was unhappy enough to call time on it, and should I just stay together for the children, I had a similarly indifferent partner, who though he professed to love me was actually so unaffected by all the things I was said about being unhappy etc that when I did end it he was completely shocked.
Now I am out of the relationship, I am so much happier. You can't spend the rest of your life in an unhappy situation. It affects you in a millions ways that you probably aren't even noticing, but its there and your children will be picking up on a sense of something not being quite right.
One thing that I possibly do regret is that I left things until they were at breaking point. I think that was mainly because my STBX just wouldn't engage with any discussion about the relationship. So instead I told him it was over and then we had some very difficult weeks of him not coping with things and behaving in a way that was very difficult to deal with. Maybe if I had been more assertive about forcing the issue sooner, I might not have been quite so driven to end it quickly (if that makes sense?).
I have realised that as far as the children go that the point of parenting is to teach children how to be good adults, and they will be looking at you and noting what you do in any situation as what they should do themselves if the same happens, thats what children do isn't it. So they will be looking at your relationship and thinking thats what a relationship should look like, and probably then if they are in the same situation dealing with it in the same way. I realised that I had to raise the bar a little higher and think about what I would want them to do if they were ever in my situation, and then give them that example.
Splitting up doesn't have to ruin their world. If your H is a good dad then you should be able to work together to find something that will work for the children.
Also, in terms of security, you may think that is what your children have now, but they don't. They are at high risk of a major change in living circumstances and will remain so all the time you are together, unless you somehow fall back in love with your husband. And when will you split up? When the youngest is 18? They will still be upset - so stay forever? You can't spend the rest of your life like that.
By breaking up you can actually promise your children with a long term certainty that you can believe in. My children are the better for the split. They were upset initially, but now they are happier (they were happy before btw, not unhappy). I think that they are happier because they have certainty now, as they know exactly what the situation is, and I think can see how sincere STBX and I are in our wish to prioritise them. That was a relief to me as I felt that there is a kind of low level dishonesty in being in a marriage that I didn't want to be in. They also have a better mum, as I have a lot more to give them in terms of parenting.
Good luck. Its a jump to leave a long term relationship, especially when there are children, but for me now looking back my overwhelming emotions are of happiness and a total relief that I got out.