The schoolboy was with his twin brother on a canal boat belonging to their nanny James Bolton when the strong magnet on a string dragged up a rusty, muddy lump of iron.
The twins, originally from Italy, quickly realised it was a grenade and passed it to James, who shuffled them off the boat in London's Grand Union Canal.
James carefully placed the unexploded device on an empty stretch of towpath.
He called the police who brought in army experts to take it away for disposal, 'Daily Express' reported.
Experts confirmed it was a World War II grenade, but deemed it safe enough to carefully take to another location for disposal.
