You always want to be honest with children, no matter how bad the truth is. Only tell them what they need to know - don't go into details. Above all, never speak ill of your ex. It will backfire, as the kids become defensive of their other parent.
Children want to know how a divorce will impact them. Some of their questions would never occur to us, so it's important to give them a safe place to ask. Listen to their concerns, and repeat back what they tell you so they feel heard.
The big temptation with kids is to talk them out of their feelings. It is so hard to see them hurt, we just want to take away their pain. But what they need is a parent who will contain their feelings. We do this by listening, repeating back what they say, and letting them know we understand. That way they aren't alone in their pain.
You might find my blog helpful. It is linked at my profile. I've written some posts about divorce and children.
Peace to you.