There is no benefit in telling the children about the situation especially when they are very young. Avoid openly speaking of the problems in front of them. If anything of that nature is required, then you can speak in general terms, such as saying to the one who asks you: There are some circumstances that prevent us from getting along, or that prevent us from living together, and other such excuses. When the children grow up and reach an age where they are able to understand that, then they will understand what is happening, and that such things – and worse things – often happen in life. So long as the matter has not reached the point of divorce, then who knows? Perhaps Allah will guide him and enable him to repent, and he will come back to you and to his children.