The teen waited for his mother and her boyfriend to fall asleep so he could unlock the restraints on their wrists, and run away.
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The detail was revealed in new court documents filed against their mother, who is waiting to be extradited from Louisiana to Harris County.
Their mother, Zaikiya Duncan, 40, and her boyfriend, Jova Terrell, 27, are being charged with injury to a child in Harris County.
They were taken into custody Tuesday in Baton Rouge after fleeing with five of her other children and triggering an Amber Alert.
Duncan appeared in a Louisiana courtroom earlier on Thursday in a straight jacket.
Bond for Duncan and Terrell has been denied.
PREVIOUS STORY: Harris Co. has 30 days to extradite mom currently in Louisiana accused of starving kids
The teens' brave getaway stunned the community where they lived, but they were able to find sanctuary at the home of a complete stranger.
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"They did not hold a single thing back. The mom had them in handcuffs in the laundry room. They were not allowed to eat," the neighbor who took them in said, adding that all the boy could think about was food. "He was saying he's starving."
Now, days after their escape, we're learning more about what they endured in that home.
In an interview with detectives, the 16-year-olds said they were forced to drink bleach, had corrosive oven cleaner sprayed in their mouths, and were routinely beaten and burned.
During the interview, police say the girl was wearing a gray plastic grocery bag around her neck. She said that's what she had to use as a shirt.
Court records also state the children said all they were given to eat were mustard, relish, or bologna sandwiches. They say they had to drink from a water supply valve for the washing machine.
When it came to sleeping, records state the teens said they were forced to take dangerous amounts of pills.
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All of this had become a routine. But this was not the only incident.
About 10 years ago, their mother was arrested for cruelty to a juvenile in 2012, in their hometown of Baton Rouge. She took a parenting class as part of probation, and the charge was reduced.
Court documents add that the abuse has been going on since at least July.
The children are now in CPS custody.
When KTRK-TV reached out to their biological father, his wife said they had no comment.