Harry Dunn's mum admits the US refusing to send Anne Sacoolas back to Britain is a blow "but one we expected."
The American, 42, fled before Northamptonshire Police charged her with causing death by dangerous driving in connection with the crash which killed 19-year-old Harry near RAF Croughton.
But the US Government last night (Thursday, January 24) officially refused a British Government request to have her extradited.
Charlotte Charles told Sky News: "Their decision that came last night is not going to stop us carrying on.
"We've got lots of other things to focus on.
"Number One will always be to get her back but it's not the be-all and end-all at the moment.
"It is a blow but it's one that we expected."
Mrs Charles learned of the American decision to turn down Britain's extradition request in a late-night phone call from the Dunn family's lawyer Radd Seiger on .
Harry's mum and dad Terry Dunn launched their Justice 4 Harry campaign after the Cobblers fan, 19, died in a head-on collision involving his motorbike and a car near RAF Croughton in August.
Mrs Anne Sacoolas, 42, the wife of an American intelligence officer at the base, flew back to the US claiming diplomatic immunity while Northamptonshire Police were still investigating the incident.
US Secretary of State Mike Pompeo last night formally rejected the British extradition request.
Foreign Secretary Dominic Raab has called US Ambassador Woody Johnson to "express the Government's disappointment about the US extradition decision on Harry Dunn’s case."
Mr Raab added: "We feel this amounts to a denial of justice and that Anne Sacoolas should return to the UK.
"We are now urgently considering our options.
"I also explained that the UK would have acted differently if this had been a UK diplomat serving in the US.
"I emphasised that work to improve road safety on and around the Croughton base must continue, and the need to resolve the issue whereby family members at RAF Croughton are immune from criminal prosecution."