I have a 4 year old son and a six year old daughter. They see their mother three weekends per month. I have them the rest of the time. I'll share a few things that I have experienced with my kids, in the hope that it might help in some way.
1. The way I chose to explain it to my kids was that mommy and daddy didn't love each other as husband and wife any longer, but still loved each other as friends (bit of a stretch there). It wasn't anything they did, etc. it was all mommy and daddy's responsibility, but nevertheless, that's the way it is. They seemingly took it in stride.
2. I love my kids and do everything to be a really good father. In fact, my only real goal in life right now is to be a good father. But, even with that, when something upsets my six year old, (you know, something like, I say she has to take a bath and she says she doesn't want to and things start to spiral downward and get out of control) she has gone as far as saying that she hates me, wishes she had another father and wants to live with mommy and have that be her family. So, just be prepared and just be a good parent and don't take it personally.
3. I let them call their mother whenever they want to, so they know that if they ever miss her, they can always talk to her on the phone.
4. When they say, "I miss mommy", I reassure them that I understand. Yes, of course you do. You'll see mommy on Friday? Would you like to call her right now?
5. I have learned that I cannot even breathe a vaguely negative comment about my xWW in front of my daughter. It bothers her when I do. For example, my xWW made cookies and sent them home with the kids after the weekend. My daughter got one out for me. I ate it. She asked me how I liked it. I said it was good, but a little too sweet for me. That upset her. I interpret this as her seeing any disapproval of my xWW from me, as a link to her possibly losing her mother altogether, which she obviously would not want to do.
6. I try not to worry, analyze the impact, fret, dispair, etc., about the fact that they no longer have a single family unit. Rather, I try to do my best to nurture them and love them, and I hope they will be OK in the end.
Good luck to you.