Do not lie to each other, since you have taken off your old self with its practices….
-Colossians 3:9, NIV
A lie to a child is still a lie. As Christians, we are to be people who speak truth and not lies.
This applies to children of divorce following infidelity. A lie is a lie.
Now, we can speak truth in a way that is cruel. I do not encourage such a delivery.
It would be needlessly cruel to tear down a cheating (ex)spouse with editorial comments on top of the facts. Name calling is not necessary. What they did was bad enough that we do not need to pile on to that.
You might say something like:
“Mommy decided to include a boyfriend in our marriage and refused to give him up when I asked her to stop. We cannot be married under those circumstance, Sweetie. That is why Mommy and Daddy are divorced.”
“Mommy is a lying whore. That’s why I divorced her!”
I strongly suggest to be sensitive to the age of the child. What is appropriate for a teenager might not be an appropriate disclosure for a five year old. Stick to the facts.
“Sweetie, Daddy chose to leave us and go to live with another woman. I did not get a say in those choices, and I am sadden to see how much they hurt you as well.
You see, God allows us to choose sin over the right thing, and sometimes that sin hurts others, too.”
“Daddy is a dead-beat loser who left his loving family for that skank! And he is going to burn in Hell for that.”
Remember that the cheater is still half of the child. Please be careful not to tear down that half and shame your child along with the cheater in your words.
But also do not be scared to identify the destructive, sinful behavior in the situation. That is a teaching opportunity for the child as well as a way to help them develop a healthy sense of right and wrong.