T.H.
It will all depend on if the girlfriend and the daddy are going to be living together. If this is the living arrangements then just tell them that daddy has a girlfriend and they will have a baby, and they will have a baby sister or brother. Then if they have questions let them ask. Please don't ignore the fact that your children will be having a sister or a brother out there. It is not the childrens fault or your fault that this had happened. Just make sure you and your ex-husband are on the same level of communication with the children. If their dad and the girlfriend are not going to be together and the children will be seeing their dad then let dad do so explaining to his children. Just make sure you are the safety net that the kids can fall back on and feel safe. They have been through to much as it is. Just let them know that you love them and none of this is their fault.
When they get older they will start to ask more and more questions. I hate to say this and it will be very hard because you got hurt also. Please don't talk horrible about their dad. Yes he hurt you and hurt the children but let the kids find out if he is a bad person.
My parents were married for more than 20 years and then they divorced. My dad talks horribly about my mom and to this day it hurts badly!! And I am 28 I don't think that I could handle the bad talk at the ages of 6, 8, or even older.
I wish you and your children the best!! You never know you might be able to spend time with the new baby and fall in love with them. I understand that you don't like how the baby came about. My brother-in-law done the same thing and his ex-wife (I still consider her part of the family) goes and picks up the daughter that he had while cheating on her. BIL and EXSIL have 3 children of their own. The children were young when this happened and they were upset with their daddy. But they were not upset with the baby. Yes they were upset with the new baby's mother, because she is the reason that split their family up.
Just take one day at a time. And make sure your children know that they have a Safe and Loving home! I am sure they know that!! Good luck and let us know how things go.