So sorry you are going through this.....the bottom line is your husband is rejecting them, you and I both know it. The kids will know it too. I have no patience for men that throw those lines around. My sister went through this years back, my best friend is going through it now. It's like they read the same manual....
There is absolutely no way for them to feel secure when Daddy is moving out. Daddy needs to know that and take responsibility. Daddy needs to grow up. Daddy needs to tell them he's leaving and you need to reassure them that YOU will never leave them. You are going to be picking up the pieces for years to come. My nephews are 42 and 39 and they still have issues with how their Dad left....you can share that with your husband...It affects every part of their life. If he loves them, he won't leave. That being said, I'm sure he will leave.
To answer your question, Daddy needs to tell them that he's leaving but YOU need to tell them, whether now or at some point later, that Daddy doesn't really understand how love works. Right now, just give them the facts...Daddy wants to live down the street. When they ask why, tell them to ask their Daddy because you don't understand either. They need to know that this is not normal and it is not supposed to be understood. Don't let them feel like they are stupid because they can't understand. That was one thing that my nephew told me he dealt with often in his life. At three and five, they will probably hear you but not really get it until he's gone and then questions will come up as they think of them. Your job is simply to be honest without giving them more information than they can handle.
T., I know this is terribly hard for you. God bless you in everything you do.