You’re smart and strong. Life is unfair for sure. Having that said, I don’t think this is legal. Have you talked to a lawyer? I don’t know Cali’s laws, but it’s one of the best states law-wise, no? I’d suspect they care about kids. I watched this YouTube show about a child support court in Texas, and there they consider allowance an income. Do you have any texts or other proof that your ex receives that money monthly? If you do, I’d hire a lawyer and taken the guy to court. If you have no proof, I’d be sneaky and send him texts discussing his allowance and saying the kids have extra expenses this months and get him to admit / at least not deny the $5,000 allowance. Discuss it with a lawyer though. California is a two-party consent state, so sadly, you can’t record him. But if he gets that money via a cheque, it means there is a paper trial. Have you done your research yet? $500 per month for a $40k yearly income plus $60k yearly allowance is super small. I’d also threaten to report him to IRS and demand to backdate the adjusted child support payments.