Harry Dunn's family plan to sue the Government and have referred Northamptonshire Police to the police watchdog over their handling of the death of their son.
They claim it was wrong for the Foreign and Commonwealth Office to advise police that Anne Sacoolas had diplomatic immunity after the fatal crash outside Croughton in August.
Family spokesman Radd Seiger tweeted: "Big day today for Harry Dunn’s family.
"Tim, Bruce and I off to London to meet with legal team led by Geoffrey Robertson QC as we sadly have no option other than to commence the legal phase of our campaign for #Justice4Harry.
"Not a situation of the family’s making!"
Northamptonshire Police officers are due to leave for the US to interview Mrs Sacoolas about the crash on her request after she left the country claiming diplomatic immunity.
Chief Constable Nick Adderley defended his officer's conduct throughout the investigation despite admitting to delaying telling the family the main suspect had fled.
Both the UK and US governments agree that Mrs Sacoolas no longer has diplomatic immunity, while how it applies to RAF Croughton is due to be reviewed.