In another report, the New York-based rights watchdog urged the Gulf oil powerhouse to protect all criminal suspects against "arbitrary" arrest.
HRW said it had documented the "routine" arrest in Saudi Arabia of children "for such ‘offences’ as begging, running away from home, or being alone with a member of the opposite sex."
Prosecutors can hold children for up to six months before referring them to a judge, while girls can be detained indefinitely for purported "guidance," it said.
"Judges regularly try children without the presence of lawyers or sometimes even guardians, even for crimes punishable by death, flogging, or amputation," HRW charged.
"In 2007, Saudi Arabia executed three juvenile offenders, including a 15-year-old boy who was only 13 at the time of the alleged crime," HRW said.
It said that while Saudi Arabia has "done little" to prosecute traffickers of foreign children used for begging, it "routinely arrests and returns trafficked children to countries such as Somalia or Chad where they risk recruitment as child soldiers, trafficking, and other serious abuses."
HRW urged Saudi Arabia to outlaw the death penalty against persons aged under 18 at the time of the offence, scrap laws that make girls and women "vulnerable to arbitrary arrest," end the detention of foreign children who are victims of trafficking and other exploitation, and "ensure that no child is repatriated to a country where they risk abuse."
According to HRW, the second report documents what it called "the arbitrary arrest" of individuals for "vaguely defined crimes."
It said suspects often face prolonged solitary confinement, ill-treatment, and forced confessions, and are denied a lawyer at crucial stages of interrogation and trial.
HRW said Riyadh should adopt a penal code that does not criminalise the exercise of basic human rights such as freedom of expression" as well as "enact new, and amend existing, legislation to reinforce protections against arbitrary arrest" and violations of due process.
