OK, I had to deal with telling my then 3yo that his brother was going to die. He's now 16 and over the years we have chatted about those times and how he felt and feels more recently.
He remembers events very clearly. And he is grateful for the straight forward honesty. We have a fantastic relationship now and there is huge trust.
I feel it's best to tell the truth. Use toys as props to explain, maybe cardboard boxes as your house and another as the other house. Get new toys for this because they will have meaning attached to the objects. I suggest cuddly ones.
If you can, find and talk in terms of life cycles: things that start and finish. Maybe books that have same characters and different stories.
Assure them that they are loved by you and their Dad. Assure them that there is nothing they have any responsibility for, nothing they did or said or thought that can change things.
I'm no expert, but I question the gradual departure in a covert way. To me that doesn't feel authentic. I prefer the idea of a celebration of the coupling that created the kids and the marking of a change in the way things will be. Make it an acknowledgement of the fantastic time that created them. Celebrate them and your love for them.
Personally, I feel that the shock of coming home to an empty house after 4 weeks without Dad would be an unkindness. I had the chance to warn my son and prepare him for a dead brother. I feel there's similarities.
It's not nice, it feels sickening for us as Mums to have to break what we are experiencing as bad news/failure to our innocent children; the ones caught in the middle. But it's empowering them to deal with things, to process, to come to terms.
The you that spends 4 weeks hiding a secret which you know will potentially cause hurt and confusion is not the same as the you who has faced the music and is nurturing them through into a new life style. Fact is, this has to be dealt with. You and only you and your ex can determine when and how to tell your beautiful children.
How will you feel best, most confident, most in integrity?
Who specifically are you protecting?
What outcome do you want?
How do they gain?
What do they lose?
What is best in the longer term?