I think you should address it in a way that is appropriate to your child’s age and within your Catholic values. When children are young, you can explain that God and we believe it is best for children to grow up with a mommy and daddy who love each other, live together, and are married to each other. You can go on to say that sometimes that doesn’t happen because some parents decide they don’t love each other, don’t want to live together, or don’t want to be married to each other. But we believe that when this happens, it makes God sad and often makes other people sad. You might explain what divorce is, in a very simple way.
I think it is better to explain things to children enough that they aren’t shocked if they do hear about things. Furthermore, as a Catholic parent, you will impart your values best if you start when they are small and add to it as your kids can understand things better. And never discount what they hear in elementary school from friends who have older siblings, or divorced parents, or parents with different values than you do.