Kids are asking why mom and I aren't together anymore

My daughter is 7 and my son is 5. They've been asking more questions lately about why I moved out and why we don't live as a family anymore. I try to keep it positive and tell them it's not their fault and that both parents love them but honestly I'm struggling with how to explain this in a way they understand. My ex and I split on pretty bad terms but we're trying to be civil now. Any advice on what you guys have said to your kids?