I agree, this is something that can wait two or three weeks. You might want to phrase your conversation this way: "H, I know that you want to speak to the children about our separation/divorce, but would it be possible to wait until after the holidays? Christmas is such a special time for them and this kind of news will put a damper on their holiday."
You have to put emphasis on how this will affect the children and especially this time of the year.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
