I am not an expert and don't know the perfect answer, but I think that I would just be real. If you have a huge smile on your face and are faking it, he will just think that you are doing it as part of a game to win him back or control him.
You don't want him to think that you are faking it in this situation, so therefore all of your other changes must be fake and not permanent. The good thing is that you have some time to prepare yourself. There is a lot of room between pretending you are happy about the situation and acting pathetic and begging for him to stay.
Good luck. I know that this is so hard, especially with little kiddos involved.
