It's not clear where you are in the process of contesting the Local 831's denial of your claim, but I would suggest making sure that you use their appeals process and submit your claim (including copies of the paperwork) in writing.
You may have been able to make a claim for child support arrearage from his estate after his death, but my guess is that the time period for any such claim has passed.
Answered on Feb 19th, 2017 at 6:26 PM