The un-named couple, both 32, were taken into custody last week for "constantly" torturing their 10-year-old son between 2020 and 2022.
They pleaded not guilty after being charged last week with committing "aggravated torture" and were remanded to Luzira Prison, a maximum security facility on the outskirts of the capital Kampala.
On Tuesday, prosecutors added another charge of child trafficking against the pair.
Chief Magistrate Sarah Tusiime denied the couple bail, ruling that a child trafficking case could only be heard by a higher court.
Police said last week that when they raided the couple's house, they found video footage showing that the child was forced to squat in an "awkward position", served only cold food and made to sleep on a "wooden platform, without a mattress or bedding."
The couple's lawyer dismissed the case as a "fishing expedition" by authorities, claiming they had no evidence.
"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."
The boy, who attended a school for children with special needs in Kampala, was one of three children fostered by the couple.
International adoptions have sparked controversy in Uganda. In 2020, the US government filed criminal charges and imposed economic sanctions against a US-based adoption ring that placed Ugandan children who were not orphans with families in the United States.