No, you should not bother writing to your ex’s parents. They have not shown any interest in you or the children, and were upset that you “prevented” their deadbeat son from doing what he wanted.
Unfortunately, the best way to get help in this situation is to go through the courts. That’s how it works.
If you don’t know your ex’s current address, but you do know your former inlaws addresses and phone numbers, you will need to give their contact info to the lawyer so they can acquire your ex’s address from them.
I am repeating the info that I gave you on your other question, because this gives you an opportunity in how to get some initial information from a lawyer for free, and you can even ask them about whether you should contact your former inlaws, and then make a plan on how to move forth. They can probably also direct you to a lawyer who could work pro-bono, or on a sliding scale. You need a lawyer.
There is a radio program from Los Angeles called Handel on the Law, hosted by lawyer Bill Handel, on KFI radio. The show is on Saturday mornings 7–10 AM PST.
Bill Handel also has a website where you can search for a lawyer, of the type you need, which would be in the categories of Divorce, and Family, and Children/Child Support.
You and your ex husband do not need to be in California to get the free referrals to lawyers in your area, and as I understand it, the first consult with the lawyer that you choose is free so that you can ask how to proceed.
You can try calling into the show, but you can also email Bill Handel and ask for assistance in how to proceed.
All of the lawyers on his recommendations list are vetted.
Here is the contact page:
http://www.handelonthelaw.com/ask-a-lawyer/
Here is more info about the site, and the menu is up in the top left corner:
http://www.handelonthelaw.com/about-us/