E.S.
Little children are very impressionable, so I can understand you wanting to make sure you do the best thing for them. My children were older when their father and I separated. Do the children see their father very often? If they do, you and your husband should sit down together and talk about what you want to say to the children. If you tell them one thing and he tells them another, that will only confuse them.
Overall, I'd say that you want to put the message in terms they will understand, but you don't have to go into details. Telling them that you and their daddy love them is important. Telling them that either you or daddy may seem sad sometimes is also important. You don't want to create a sense that everyone must mask their true feelings. Children are very sensitive. It's ok for you to be sad, and it's ok for them to know you are sad.
Tell them that you and their daddy have decided that you need to live apart for a little while. Tell them that it's something that has to do with being all grown up and that it is something that is hard. Tell them no one did anything wrong. No one was "bad".
I think the important thing is to communicate in their language. Reassure them, let them know they aren't to blame, and then help them understand.
I don't know if that helps you any, but I wish you the best of luck.
Take care.