Ron Smallshire, 56, attacked Michael Oakley with a steak knife after one of his Alsatians ran into his garden in Highcliffe on 16 December 2005.
Mr Oakley, 24, who was with his three younger sisters at the time, suffered up to 31 stab wounds.
Smallshire admitted one charge of wounding with intent at Bournemouth Crown Court on Tuesday.
The court heard how Mr Oakley's sisters Kimberley Oakley, nine, Heather Oakley, 11, and Krystyna Stone, 12, were "traumatised" by what they saw.
"If the dogs were fighting you can perhaps understand a reaction"
They had wanted to walk Mr Oakley's two Alsatians with him before bedtime.
The attack happened when one of the dogs ran into Smallshire's garden, as he was letting his collie dog out.
Smallshire, who worked as a technician at King's High School in Bournemouth, told the police he thought his dog had been set on by the Alsatian and he was being mugged.
Outside court, Det Con Carl Wooff, of Dorset Police, said: "It was a frenzied attack. I would say he is lucky to be alive.
"What is significant, what makes the whole thing more sinister is, if the dogs were fighting you can perhaps understand a reaction.
"However, Smallshire has left the area of any dog fight, gone back to the house and taken out a knife.
"He has then chosen to come back and launch an attack."
Smallshire is due to be sentenced in September.
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