What state law in Texas states that a signature isn't required on child support income withholding order?
The Personal Responsibility and Work Opportunity Reconciliation Act (PRWORA) (P.L. 104-193) includes provisions requiring that the State IV-D agency must have the authority to issue income withholding orders administratively. (See 42 U.S.C. 666(c)(1)(F).).
In Texas, the IV-D agency is the Texas Attorney General.
Therefore, by application of federal to Texas, the Texas AG can issue administrative writs of withholding without a judge's signature.