I have a 4 year-old and a 2 year-old. The children's father and I are splitting up..?

The children's father and I are splitting up and I am not sure how to talk to my kids about what is going to happen. My four year old is very bright and already knows something is going on even though we are still living in the same house. We do not fight at all in front of the kids, we don't really fight at all period. The kids and I will be moving into my mothers house soon. I really just need some advice on how to talk to my children and have them understand what is going to happen. I want this situation to have the least amount of trauma for the kids as possible.