Frist, I am so sorry to hear about your marriage coming to an end. It sounds as if you & your husband have thought this well out....are you certain that there is not way to rebuild your marriage/relationship???
If you and your husband are certain that this is the way that things will be, I think that it is imperative to tell your children as soon as possible.
How??? Well first off do it together. Do it in your home or a safe place where you know that you all feel is special and unites you. Explain to your kids that you both love one another and most of all them and have something very important to tell them. Tell them that sometimes adults can love one another but that may not be enough to be together without it hurting one another and sometimes it means that they can no longer live with one another. If there has been fighting/arguing point that out and tell them that you don't want to do that anymore. Reassure them more than anything that this has absolutely NOTHING to do with them in terms of something they did or didn't do, or something they said or didn't say. Explain that you have decided that mommy & daddy would be happier living apart and will have what they call as a divorce. That just because this is happening won't mean that things will change as far as how much you each love them or how much they will love you. That both of you will be just as actively involved in their lives forever
because they are yours and always will be. Briefly explain what you plan on doing such as "Daddy is going to live here until mommy finishes Nursing School and has a good job to help pay for things", and, "We are going to figure out things together and let you know what changes will occur because we are a family and will discuss with you the things that will change." Reassure them that you will try not to make too many changes such as school and friends because just because mommy & daddy are making changes doesn't mean that they have to make too many themselves and that you love them & always want what is best for them. Keep it brief and tell them that they can ask either of you any questions...that it is nobody's fault that mommy and daddy are not staying married and that you don't want them to place blame on either one of you because when a marriage ends it is usually both people's faults.
If they have questions and you know the answers answer them honestly. If not, then reassure them that you will let them know as soon as you know.
Kids respond most to honesty. I tis best that they can trust you and not have to hear this from anyone else than those that they love the most...their parents.
I wish you luck ~ Goody