I think I would leave well enough alone right now w/the FIL. There will come a time when you can open the door a bit and step thru it and have a discussion w/him about visiting w/his grandchildren.
How long will it take for the consequences of his actions? Well, it could take a week, month, year or even years...but it's not something that will happen quickly. Karma has a way of working when we least expect it, so the only thing you can do is live your life and not focus on whether the Karma train has left the station just yet.
I know you and your h would normally go to the seaside on a day like to day, but what is stopping you from going w/your children? Just because your h is MIA, that doesn't mean you can't do things on your own and yes, even enjoy them. The best revenge in all of this mess is to live your life to the fullest.
It will get better, but it takes a lot of time and patience to get there.
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.
