Backstabbing Ex doesn't have to pay support or arrears.?

here is my problem. I was a custodial parent with 2 children. I was receiving child support. My ex wife stopped paying it. The arrears eventually added to more than $8,000. She then applied for social security disability and was approved. That was when I was asked to go back to court where the judge put my child support and the arrears on hold. about 3 weeks later i received a notice from child support services telling me the case was terminated because she had no income that they could attach. I called them and was told that there was nothing I could do and that the arrears would also disappear. I also tried to get social security benefits for the children thru her SSD but she doesn't have enough work credits. She now has one of our children. and hasn't attempted to apply for child support. She will be getting a child tax credit on her tax return until the child with her turns 18. How can i attach her tax return to get payment for what is owed to me. We are in California. someone told me to start a lean. but i'm not sure how to do that. Any legal Advice would help. THANX