@Kione
This is exactly what I told my kids (my STBX was there but did not say anyting)," Dad and I are separating. We do not get along so well and over the years we tried to work it out but could not. I want to you to know that we both love you very much and always will be here for you. Also, know that this has nothing to do with you."
That is it. First remember, you do not want to go into too many details with the kids because your relationship with your husband is your personal matter, also it may turn into blaming and you do not want to do that. Also, be clear and get to the point. The more you talk the harder it gets. I literally wrote this down and learned it by heart. My kids are older, 22, 19 and 16, but they live with us sill. They were stunned, shocked, there were a few "wow"s but that is that. The youngest started crying and left the room. Tomorrow 19 and 16 years old sat down with me to ask if I though this over, am I sure, did I give it enough though? It was really painful and scary but I thought I should stay calm and be firm and clear, and said, "yes, I gave it a lot of thought, we have been together 26 years and if we were not able to make it work by now, I do not think we ever will". That was that, no more questions, nothing.
I am sure that kids are struggling with this and thinking about it and worrying, but there was no more questions. I think it is important they know that you are certain about this, they you are not questioning yourself. My theory is that as long as I am acting normal, like this is not the end of the world, like this is just a part of life, not talking bad about their dad, talk openly about our plans, him moving out, my life afterwords and stuff that it will be ok. I mention sometimes in conversations, that I am anxious about all, but I am not crying, I am not making this into some kind of tragedy, which it really is not.