The judgments handed down against the convicted men came in the northeast city of Sylhet and southeast city of Khulna. In both cases, the killings had touched off widespread, angry protests with a demand for bringing the perpetrators to justice.
The separate cases came as Bangladesh has seen an increase in child killings in recent years. Most of the victims have been poor children, and motives for the slayings often have been seemingly minor issues.
Statistics from Bangladesh Shishu Adhikar Forum, a child rights group, show that 191 children were slain in Bangladesh in the first seven months of this year, up from 146 during the same period of 2014, and from 128 in the corresponding period of 2013.
In Sylhet, Judge Akbar Hossain Mridha sentenced four men for killing 13-year-old Samiul Alam Rajon, a vegetable vendor.
"They will be hanged to death," the judge told a crowded courtroom, referring to defendants Kamrul Islam, Taj Uddin Ahmad, Mayna Miah and Zakir Hossain.
The youth was tied to a pole and beaten to death on a false theft charge on July 8, according to authorities.
A video of boy's torture was posted on social media, resulting in protests across the country.
In Khulna, Judge Dilruba Sultana sentenced Md Omar Sharif and Mintu Khan to death after they were convicted of killing 12-year-old Rakib Hawlader.
The boy died on August 3 after air was pumped into his body from an air compressor used for inflating tires.
Sharif had employed the youth in a motor workshop, and Sharif became angry when the boy took a job at another shop, authorities said.