Does a chapter 13 bankruptcy plan in Houston have to pay 6% interest on child support claims?

Yes.

U.S. Bankruptcy Judge Jeff Bohm ruled in In re Lightfoot, Case No. 13-32970, SD Texas 2015, that child support arrears being paid through a chapter 13 bankruptcy plan must be paid in full and to be paid in full, they must in addition be paid 6% interest according to the Texas Family Code.