Glen Edwards, 18, from Ickenham, died after the Vauxhall Corsa he was travelling in was hit from behind by a car driven by 38-year-old Matthew Driscoll.
Last week, Driscoll, from Colchester, was fined £530 and disqualified from driving for six months after pleading guilty to careless driving at Chelmsford magistrates' court. Glen's father, Andy Edwards, 50, said: "When I heard the sentence read out in court I felt like I had been hit with a sledge hammer. All the wind was knocked out of me. I couldn't believe it. "He [Driscoll] killed my son and destroyed my life and it cost him £530 and a six-month driving ban.
"The whole situation is disgraceful. The Government has said it will introduce a new offence of causing death through careless driving - it should do it as soon as possible." The Ministry of Justice said no time scale for the introduction of the new law had been set.
"He is an absolute coward. He ran over a four-year-old boy then ran away."
Casey was crossing the road with his five-year-old sister Lillian after playing in a park near his home in Lancaster on July 17 when the Renault Megane hit him. He died later in hospital. When Diprose appeared before magistrates he admitted driving without due care and attention, failing to stop and report an accident and driving without a licence or insurance.
Yesterday, he was a free man after a court said it had no power to jail him because his driving had not been adjudged dangerous.
Casey's parents described the outcome as "a total insult. If he had stolen money, he would be behind bars now," said Mr O'Callaghan, 44. "The sentence is barbaric. Justice wasn't done today. It is beyond belief.”