I am very sorry to hear that your father passed away. It's never easy to watch a parent slowly leave this earth. Right now, your focus will need to be on the funeral and supporting your children thru this sorrowful event. Please take care of yourself.
As for your xh, the next time he calls demanding that the kids call him, tell him that he needs to deal w/them directly. They are old enough to make decisions about whether or not they want to speak w/him.
He really needs to call 1-800-Stupid. What an insensitive @sshole.
I will keep you and your family in my thoughts in prayers.
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.