Miniature caskets held Noah Pozner – the youngest victim of the tragedy in Newtown, Connecticut – and Jack Pinto, also six, as they were laid to rest.
In all, 20 children aged six and seven were shot dead, along with six adults at Sandy Hook Elementary School.
Police and bomb-sniffing dogs conducted a precautionary search of the street in Fairfield, Connecticut, which housed the funeral home where Noah's brief life was remembered.
The area outside the funeral home was lined with white balloons.
A teddy bear and white flowers lay at the base of an oak tree outside the Jewish service, which was packed with tearful mourners, including Connecticut Governor Dannel Malloy, American Senator Richard Blumenthal and Senator-elect Chris Murphy.
In keeping with Jewish tradition, Noah was laid to rest in a simple brown wooden coffin adorned with a Star of David.
"Every time I told Noah, 'I love you,' he always answered, 'Not as much as I love you,'" his mother, Veronique, told mourners.
In Noah's obituary in the Newtown Bee newspaper, his parents and four siblings said, "Noah was an impish, larger-than-life little boy. Everything he did conveyed action and energy through love. He was the light of our family, a little soul devoid of spite and meanness."
At Jack Pinto's funeral in Newtown about a half dozen children wearing a wrestling club's gold medals took off the awards and gave them to their team-mate's parents.
Jack was a New York Giants fan and was wearing a red-and-white jersey with receiver Victor Cruz's number 80 as he lay in an open white casket at the service. During Sunday's game, Cruz wore shoes with "R.I.P. Jack Pinto" written on the side.
In his obituary in the local paper, his parents Dean and Tricia said: "Jack was an incredibly loving and vivacious young boy, appreciated by all who knew him for his lively and giving spirit and steely determination.
The boys were buried a day after the small community of Newtown, already taking down Christmas decorations, came together for a vigil attended by US President Barack Obama.
Mr Obama said he would use "whatever power" he has to prevent similar massacres.
But he has given no specifics on what he might do, and White House spokesman Jay Carney warned "no single piece of legislation or action will fully address the problem."
Officials could not say if the school where the killings happened, Sandy Hook, will ever reopen.
Nervous students and teachers across America returned to classrooms under tighter security.
Before the two funerals, police in the town of Ridgefield, about 20 miles from Newtown, investigated reports of a suspicious man who could be armed. Officers ordered a lockdown of two schools.
But after a few hours, police said they had found no dangerous activity and ended the lockdown. A police presence remained at each school.
Investigators are still looking for a motive for the shooting, and the Connecticut community is struggling to comprehend what drove 20-year-old Adam Lanza to carry out the massacre.
He shot dead his mother at home in bed on Friday morning, drove her car to the school and then opened fire, killing 26 people and injuring others.
All the victims were shot more than once, and some of them were shot at close range, said Chief Medical Examiner Dr H Wayne Carver,
He said the ammunition was the type designed to break up inside a victim's body and inflict the maximum amount of damage.
Further details on just what happened during the shooting were "too difficult to discuss," said state police Lieutenant Paul Vance.
He also said police may hold the school and the Lanza home for months as the investigation goes on.
Police said Lanza was carrying enough ammunition to kill just about every student in the school if given enough time.
He shot himself in the head just as he heard police drawing near, authorities said.
Investigators believe Lanza attended Sandy Hook many years ago, but they could not say why he went there on Friday.
Authorities said Lanza had no criminal history.
Divorce paperwork made public shows that Lanza's mother had the authority to make all decisions regarding his upbringing. It makes no mention of any mental health issues regarding Lanza.