It’s important that you use language that’s right for their age. You don’t need to go into detail about the reasons why you are splitting up with a young child, but you do need to reassure them of your love. However, if they are older and aware of the issues, tell them openly - but only give them the amount of detail you think they can handle. Don’t underestimate their ability to understand. Chances are they’ll already be aware that something is going on, even if you have tried to keep it from them.