Surgeons who spent hours operating on Tim Parry said his skull had been fractured and most of his face blown away.
Johnathan Ball, aged three, died in the attack on Saturday afternoon in the Cheshire town. Fifty-six shoppers were injured, many seriously.
Colin and Wendy Parry, from Warrington, told reporters outside the town's hospital that Tim, a keen Everton supporter, had been to buy a pair of football shorts with two friends.
Mrs Parry, eyes brimming with tears, said: 'The first explosion frightened him and he ran straight into the second one. Tim took the full force of the blast. He had been in surgery for hours before we found out.' He has not regained consciousness.
A weeping Mr Parry said: 'I have got a son who is not going to live, a good-looking 12-year-old boy pulled apart . . . and for what? I just feel empty.
'I have no words to describe them. We are losing a son we loved, that's what matters. I don't know what motivated these people. They are inhuman, wiping out a life and ruining a family. They are taking away the boy we loved.
'To do this in the main shopping area on a Saturday filled with so many people . . . I have got no words . . . I can't describe them. They are just inhuman callous killers.'
The IRA yesterday admitted causing the explosions and said republicans would 'profoundly regret' their tragic consequences and blamed the authorities which had 'deliberately failed to act on two precise warnings 30 minutes beforehand'.
Cheshire police said there had been one call to Merseyside Samaritans telling them about a bomb outside an unspecified branch of Boots in Liverpool - making no reference to Warrington, 30 miles away.
Kenneth Clarke, the Home Secretary, said the IRA claims were 'lies' and dismissed their expressions of regret as 'hypocritical'.
A teenager who occasionally looked after Johnathan Ball told how the boy's father, Wilf Ball, searched for his son after hearing of the explosions.
Rebecca Higgins, 16, said that Johnathan had gone to town to buy a present for Mother's Day with a family friend. 'Wilf was just walking round in a daze, hoping Johnathan had got lost in the panic and would come running down the street any minute.'
Channel Four postponed last night's premiere of Hidden Agenda, a film by Ken Loach set in Northern Ireland.