Jennifer Richards, 25, also allegedly was plotting a similar fate for another daughter, 10-months old.
Richards escaped trial with her guilty plea after federal prosecutors dropped two charges against her but she still could get 20 years to life, the San Antonio Express-News said.
She also could be called to testify against her reputed partner, former boyfriend Sean Michael Block, 40. He's charged with aiding and abetting Richards and distributing child pornography. His trial is set for May 26.
Authorities said the children are no longer in Richards' custody and that neither child was actually sold for sex.