The matter whether the trust is revocable or irrevocable does not control in child support cases. It is actually very easy to find out whether it is recovable without even knowing the trust instrument by asking the question, who pays for taxes?
The courts in California imposed child support on irrevocable trusts before. See Ventura DCSS v. Brown as trustee. A spendthrift trust by definition is irrevocable. The same as minor's trust or support trust. However, the California courts do not hesitate to collect child support from these trusts. See Lakenan v Lakenan. Right or wrong, that is the state law.