Trusting, I am very sorry the loss of your xmil. It has to be a shock to everyone that this happened so quickly and unexpectedly. I'm sure your xfil is at a loss right now.
As for your xh, this is not the time or place to be discussing child support and you did the right thing by advising him that you will not discuss it.
My thoughts and prayers are with you and your family at this time.
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.