Hannah Saaf, 28, was ordered to be detained indefinitely in a secure mental hospital by Bristol Crown Court yesterday.
The mother of twins and law graduate was driving at 60mph when her car mounted the pavement and knocked down Sam Riddall as he walked home from a youth service at the Henleaze and Westbury Community Church in Westbury-on-Trym, Bristol, in May. Mrs Saaf fled the scene but was caught nine days later hiding in a Somerset barn.
Judge Simon Darwall-Smith told Mrs Saaf that she would have received a tougher sentence had it not been for her mental state.
"You were clearly under the influence of cannabis and you were more than twice the speed allowed," he told her.
"You then fled and left the scene having killed the victim.
"There is no sentence that any court can pass which will ever compensate for a tragic loss such as this.
"But for your mental state this would have called for a very substantial custodial sentence but I am satisfied on the evidence of two psychiatrists that a hospital order is the only feasible sentence in this case."
Speaking outside court after the sentencing, Sam's father Martin Riddall said he and his wife had forgiven Mrs Saaf for killing their son.
"In four days we will celebrate Christmas for the first time without Sam," he said. "It is going to be very hard for us indeed because we still miss him very much.
"At Christmas we remember God sending Jesus into the world to bring peace.
"It is this same God that is giving us the strength and helping us to forgive Hannah for the terrible thing she's done to us by killing our son.
"It's not easy, it's not going to be easy, but with God's help we know that it is possible."
Mr Riddall, who works for Campus Crusade for Christ International, went on to thank the couple's friends at church and in the community, as well as the police for their support over the last eight months.
Mrs Saaf has a history of mental illness. She was diagnosed with bipolar disorder in 2003 and has been treated in mental hospitals on several occasions. She said in a statement to police that she had "stolen a life" and felt "deep remorse" for causing Sam's death.