I would suggest that you both sit down w/the children and tell them together. If that can't be done, then he needs to speak to them and not leave the entire situation in your hands. I would suggest that you tell the children that "dad's going to be moving out for a while because he has some things that he needs to work on for himself." You do not need to go into detail about why or for how long. If they ask how long, you can always say for a while.
It's best to reassure them that they didn't do anything to make dad unhappy and that he loves them.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
