The mother of Harry Dunn said it was "cowardly" for the woman convicted over his death to not appear in a UK court.
US citizen Anne Sacoolas was sentenced, via video-link, to eight months in prison, suspended for a year, for causing death by careless driving.
The Old Bailey heard the US government had said "her return could place significant US interests at risk".
Her legal team have been contacted for comment but she previously said she was "willing to meet" Mr Dunn's family.
Sacoolas was driving on the wrong side of the road when she hit the 19-year-old outside a US military base in Northamptonshire in 2019.
Following the crash, the 45-year-old had diplomatic immunity asserted on her behalf by the US administration and left the country 19 days later.
She never returned to the UK, leading to a transatlantic row between the US and UK governments.
Speaking to BBC Radio 4's Today programme, the teenager's mother, Charlotte Charles, said the family was "really OK with the verdict".
"We were told we had a less than 1% chance of getting anybody held accountable for this," she said.
"We did not take that lying down, we came out fighting from that moment on and three and a bit years later we have that conviction."
After Sacoolas pleaded guilty in October she was told by the judge to appear in person for her sentencing.
But at the Old Bailey on Thursday, Mrs Justice Cheema-Grubb said she received a statement from the US government that said it "does not in any way support Mrs Sacoolas' appearance at this hearing".
Mrs Charles said it was "totally cowardly" for Sacoolas not to appear in person.
She said the US citizen "should have found the strength" to return to the UK.
"But I can't change that, she's a very different person to who I am and my family are, unfortunately [she is] a really large coward," she added.
'Time for us to grieve now and time for us to start celebrating Harry'
Harry Dunn's mother Charlotte Charles spoke to #BBCBreakfast after Anne Sacoolas was sentenced for causing his death as he rode his motorbike in 2019 https://t.co/YFpd2C0SM2 pic.twitter.com/NwfCxmZYzD
The family's campaign for justice at one point led them to the White House and a meeting with the then-President Donald Trump in October 2019.
Mrs Charles said it was "surreal" and that "it doesn't even feel like it happened".
During the meeting, Mr Trump revealed Sacoolas was in the next room, but the family did not want to meet her.
"It was totally and utterly the wrong thing to do and when we said no to that stunt we were promptly shown out the back door of the White House," Mrs Charles said.
In a statement issued on Thursday, Sacoolas said she was "deeply sorry for the pain that I have caused".
"As always, I remain willing to meet and apologise to Harry's family directly if that would support their healing," she said.
The statement said she was "grateful that I could express my profound remorse to the court and to Harry's friends and family".
She added that the day of the crash "changed the lives of so many, and I pray for healing".
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