One of the ways to tell your kids you’re getting a divorce is to tell them that you or your spouse isn’t to blame. Do not say anything bad about one another to avoid making your children feel resentful or bitter towards their other parent. Even if your spouse has committed adultery or abuse, your children don’t have to know about it.
You can try to say something like "Mummy and Daddy have grown up issues with each other. And these issues cannot be fixed. We have decided not to be married to each other anymore, but we still love you very much. While some things may change because of this, there are some things that will never change. Your mummy will always be your mummy and your daddy will always be your daddy - and we will always love you, no matter what."