Ms Mackenzie Leing Mathias Spencer and Mr Nicholas Spencer are accused of administering harsh punishments to the child.
Ugandan prosecutors on Tuesday charged a US couple with child trafficking and denied bail, a week after they were accused of torturing their foster son.
The couple, both 32, were arrested last week on suspicion of "constantly" abusing their 10-year-old son between 2020 and 2022.
They pleaded not guilty after being charged with "aggravated torture" last week and were sent to Luzira Prison, a high-security prison on the outskirts of Kampala.
Prosecutors added another accusation of child trafficking against the two on Tuesday.
The pair were denied bail by Chief Magistrate Sarah Tusiime, who ruled that a child trafficking case could only be addressed by a higher court.
When police raided the couple's home last week, they discovered camera footage showing the child being forced to squat in an "awkward position," provided only cold food, and forced to sleep on a "wooden platform, without a mattress or blankets."
Authorities' "fishing expedition," according to the couple's lawyer, was a "fishing expedition" since there was no proof.
"Last time we were in court, the state said that inquiries are complete and yet today they added a new charge and said that inquiries are ongoing," she told AFP.
"It doesn't make sense", she added.
The boy, who attended a special needs school in Kampala, was one of three children fostered by the couple.
The couple moved to East Africa in 2017 to volunteer at a US-based non-profit in Jinja before moving to Naguru, a posh Kampala suburb, to work at a start-up.
International adoption has raised debate in Uganda.
In 2020, the US government brought criminal charges and issued economic sanctions on a US-based adoption network that matched non-orphan Ugandan children with families in the US.
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