Be kind - this is probably harder for your children than it is for you. You need to be honest with them for sure; but the way you report this news to them will impact them forever so you need to be as gentle as you can. Tell them that their father will not be staying with them as he used to before. I don't think you need to tell your little ones about the 'other woman' as they surely won't be able to comprehend what's going on. In time, however, things will get clear to them. Be careful not to speak ill about their father to them..... give it time. Good luck!
It is perhaps one of the most difficult things a mother/father has to do. Further, it becomes more difficult in your case since your husband is leaving you for another woman. Such news has the potential to influnce a child's life and psychology. You must break the news with the10 year old kid. Though you can wait for the right age to inform other children. You must not impose a negative influence of the father on the kids. You should be positive about this thing. You may tell them that the only thing that's going to change in future is less often visits by your father. You may tell them that he is not going to live with the family any more. There can be other ways but then the only thing you must take care of is that the kids don't grow up hating your husband even though he deserves it.