Eight-year-old Martin Richard had been waiting with his mum, dad and two siblings to cheer on runners at the finish line when the bomb went off.
Martin, from Dorchester, Massachusetts was one of three people who were killed by the blast and his mum, dad and sister were amongst 144 injured.
Mum Denise, 43, has undergone brain surgery and six-year-old sister Jane lost a leg in the blast,. His dad Bill, 42, suffered shrapnel injuries to his legs while Martin's older brother, Henry, 12, escaped injury. Although he is a runner, Bill had been injured and did not compete in the marathon.
The family had just returned to their seats in the grandstands after buying ice creams.
In a statement, Martin's dad Bill, 42, a community organiser, released a statement, saying: "My dear son Martin has died from injuries sustained in the attack on Boston. My wife and daughter are both recovering from serious injuries.
"We thank our family and friends, those we know and those we have never met, for their thoughts and prayers.
"I ask that you continue to pray for my family as we remember Martin."
It was revealed that last April, Martin had posed with a sign saying: 'No more hurting people', and 'Peace' in a poignant photograph. He was pictured after learning in class about the death of Trayvon Martin, a black US teenager shot after he got into an argument with a Neighbourhood Watch co-ordinator.
Dan Aguilar, 50, a project manager who lives next door, said the Richards seemed to be the 'perfect happy family'.
He said: "I saw them on Easter Sunday all sitting on the porch as a family and they all looked so happy. The boys were always playing in the yard, Martin seemed so full of life.
"They were just a typical American family. When I heard the news it kept me up at night. Their little boy will never come home again."
Among the other victims of the bombings were two brothers who both lost legs. JP and Paul Norden were standing next to Martin Richard when the bombs went off.
Paul Norden, 31, whose girlfriend was also seriously burnt, rang his mother, Liz, from the back of an ambulance.
"Ma, I'm hurt real bad," he told her.
His 33-year-old had also lost a leg in the same spot - just below the knee - due to serious burns.