P.M.
We all change over time (this can be with or without new wisdom and maturity). Couples tend to grow in different directions, unless they have made a conscious and consistent intention to stay in sync, and even then differences can set in. Simply put, our needs change as we grow older, and what we want, need, and expect in a partner can look drastically different from our new perspective.
And then the whole parenting thing can be a huge wrench in the works, too. Marriages tend to work best when you've talked through as much potential disagreement as possible well before you have children, to make sure your parenting styles will mesh well.
But even with the clearest of intentions and most careful preparation, there is simply no way to guarantee that a marriage will still fit well 12 or 18 or 25 years in. Life has a way of marking us all.
You are right to be concerned that divorce will be hard on your children. It will not necessarily "tear them apart," however. There's a very compassionate book called Raising an Emotionally Intelligent Child, by John Gottman. Studies show that children whose parents practice "emotion coaching" are more physically healthy and emotionally resilient, less affected by stress, perform better academically, have better relationships, and are less likely to develop behavior problems. This is true even in the case of divorce.
I hope you'll try counseling first – some people find new life in a dying marriage. Whether or not that succeeds, I strongly recommend you learn emotion coaching for your childrens' welfare. You'll find the techniques benefit you, as well.