It sounds like you've already thought of the basics, such as telling them together. If he's been sleeping on the sofa do they already know that something's changed? If you are different towards each other have they picked up on that?
I think you can still tell them that mum and dad are not going to be together any more even if he's not moved out. You could say that dad is looking for somewhere new, talk it up as a new place for them to visit, have some of their own stuff there etc.
Really important to keep stressing that they are not part of the split. They're very young, and you may get some very random questions asked of you. I would recommend short, to the point statements. Try and emphasise how much will stay the same: where they now live, current school, same friends, all family still the same, still see dad regularly.
Don't try and pretend it's not sad, that you're not sad, and make sure they know it's ok to be sad. Basically it'll be tough, whatever you say, but try and be calm, factual and reassuring. Good luck.