Sonny Cater was exploring fields near his house when the National Geographic metal detector, worth £30, led him to a mud-caked metal capsule.
He took it home but his father Jem, 37, became suspicious as he washed it under a tap and contacted a relative who is a former RAF armourer.
The family were told to immediately call the police and a bomb disposal squad was dispatched to their home in Kings Lynn, Norfolk.
Experts identified the device as a 10lb British practice bomb from WWII and it was removed for safe disposal.
Yesterday his mother Tracey said that, despite the drama, the incident proved that Sonny's Christmas present worked.
She said: "We are dumbfounded that he discovered this on his first go.
"We are going to go out again to see if he can find something Roman. It has made our Christmas.
"It was caked in mud and Jem just thought it was a lump of metal and took it home.
"Sonny did become a little nervous with the arrival of the emergency services."
Sonny was enjoying a walk across Roydon Common for around 15 minutes with his parents and brother Marley, nine, on Boxing Day when his metal detector started beeping.
He dug up the treasure but couldn't make out what it was – so he hurriedly bundled up the muddy object and took it home to wash down.
Mr Cater contacted his partner's father, Steve Wood, after uncovering the pointed end.
Mr Wood, who had served more than 20 years in the RAF armoury, advised him to call 999 and place it in a bucket of cold water.
This was a precaution in case it was a German phosphorous bomb, which would ignite if dry.
Bomb disposal experts from RAF Wittering in Cambridgeshire rushed to the family home and identified the item as a 10lb British practice bomb head.
The bomb head still contained internal wiring and was taken away for disposal.
It is believed to have been used in practice World War II bomb runs.
Luckily the 10lb bomb head did not contain any explosive material.
His mother, 39, a nurse at the Queen Elizabeth Hospital in Kings Lynn, Norfolk, said: "Kids always love looking for treasure so we thought it would be a fun random present for his stocking.
"When the it started buzzing we all thought it would be some two pence pieces or something like that – I never thought it would be anything this serious.
"It was all very exciting, the kids and Jem started digging and then our crazy dog started digging too.
"It was a big muddy lump when it came to the surface so we stupidly thought lets take it home.
"We feel a bit silly now we know it could have potentially been dangerous but its not often we go exploring and end up with a bomb in a bucket of water at the end of the garden.
"I should imagine there was a few curtains twitching on our road on Boxing day.
"There was the police and bomb disposal outside our house the neighbours must have thought we were mad."
The torpedo-shaped bomb which was discovered was of no danger to the public but was deposed of.
Practice bombs were used in the first and second world wars to allow military to practice without causing the same excess of damage as they would with a regular bomb.
Training pilots would also use the 10lb practice bombs as they were cheaper than the £1000 worth of bomb used in military attack.
Flight Lieutenant Donald Earl, an RAF Wittering spokesman, has advised people to call police and not move suspicious items.
He said: "We find a lot of bombs in Afghanistan with metal detectors but we don't tend to find them in the UK.
"We would urge members of the public to leave suspicious items in situ."
