I think with kids that age you need to present things as "we". Even if it isn't, and you don't believe it. My opinion that the kids have to see their parent as a unit, for parenting purposes. Even if you are divorced, the kids have to know that when it comes to parenting, they can't play one against the other. In time, they can be told more background, but not at 4 and 7. And I don't think I would mention the judge at all, they are going to think that a judges job is to break up families.
You don't want to lie to the kids, so say Mom and Dad can't live together right now. That is the truth, you don't need to go into why, and assigning the blame. Once you start going down that road, there could be enough dirt slung that the kids will hate both of you!
