My question is this can child support be ordered on someone who only receives SSI as income? And where would I find the law about this for Texas? If anyone can help me with this I would be appreciative.

Yes it can, it is also one of the few exceptions to garnishment of a social security check. Go to the socialsecurity.gov website under frequently asked questions.

Frankie

Section 207 of the Social Security Act (42 U.S.C. 407) protects Social Security benefits from assignment, levy, or garnishment. However, the law provides five exceptions:
Section 459 of the Act (42 U.S.C. 659) allows Social Security benefits to be garnished to enforce child support and/or alimony obligations;

So yes they can take it from your SSI become it is consider your income.