Telling the children, tip 1
It's not your fault!
Children, particularly under 8's can internalise their feelings and worry that they have caused family breakdown. They may feel that if they had been more helpful, been better somehow this would not have happened. Reassure, reassure and reassure.
It's possible that younger children will think these blaming thoughts but not articulate them. Be watchful for withdrawn behaviour, sudden emotional outbursts or uncharacteristic anger or unkind behaviour to others. Try not to blame each other either. Present a united front for their sake if you can and try and find some practical positives for them about the changing shape of their family.
Encourage your child to talk, listen to their concerns and let them feel that you are as emotionally available for them as you always were.