How do I explain why daddy left, If i don't completely understand?

My husband left me and my children on a Sunday afternoon, 2 weeks after our 10th anniversary. He had been acting weird for a month or so, going to the same bar night after night in our very small town. Hundreds of dollars at a time he came home drunk and more distant. One Saturday morning he sat me down and told me he almost slept with someone. he said she came on to him and he stopped it before it went too far. I thought that was complete crap, but i listened and kept it in the back of my head for the next week of our anniversary, and our trip away. We had a beautiful anniversary flowers at work, dinner and a bed and breakfast getaway. He was distant towards the end, and I knew something was up. That next weekend, I decided to go out and see for myself. She was there, and all night she was stuck to us acting like a total bar tramp. He made many comments about how nasty she was being, but something didn't feel right as he danced with her and sat by her all night. 2 days later he went out and I found them together again at 7 am in our small lil town truckstop, that afternoon he left, and hasn't been home since.3 days later I found out that she left her husband and son and was staying with him in the hotel. It has now been 3 weeks and they are in love, he lives with her son and her at her friends house, while they apartment hunt. He has barely seen the kids other than to drop off money for bills, and this weekend he insisted on taking them for an overnight to meet their new step mommy. They missed their daddy and since legally i couldn't stop him, i had to let them go.Luckily they had a good time, but have so many questions. yes I filed for divorce, and he is thrilled, but I knew it was too soon for the kids to understand, and now the insanely hard questions have started, and the wanting daddy to come home. I have them in counseling in school and I have started counseling, but I just don't know what to say to explain. There are so many more insane details, and if I am still in shock and disbelief, how do I explain what daddy did and why he isn't coming home????