This when groupthinking really shines. Different versions of your message, while keeping the spirit and intent the same. Some minor changes and a few tweaks until you are satistified. It is not to optimize the wording - it is for you to figure out and understand what you really are trying to say to your kids.
I’ll start.
A good message, with good sprit, maybe deliver it before Christmas not on Christmas. Try not to have bad news on Christmas Day.
From what you have wrote I would accent the following thoughts from your original.
Make the message really about their view, how it affects them, the big focus is that you love them and they are not at fault. Remember kids think the world revolves round them.
I would also suggest for the time apart sentence using need instead of want. I do not think you want to be apart, I do however think you see a need for the two of you to be apart. Your kids will not see all the subtleties of this but they will see needs vs want.
Kids, your stepmom and I are having some problems and need to spend some time apart. I want you to know that you are not responsibly in anyway, our problems are strictly between us. You kids are the best and most important people to us and we both love you very much. I know this is upsetting for you. I am here for you and you can talk to me any time you want. I love you very much.
DnJ
Feelings are fleeting. Be better, not bitter. Love the person, forgive the sin.