The sentence handed down to Baby P's mother today was condemned as "disappointing" by a children's charity.
She received an indeterminate jail term at the Old Bailey but will be eligible for parole as early as August 2012.
The NSPCC said this meant she could walk free from prison at a time when her son should have been in his first years of school with his whole life before him.
The charity also criticised the sentence given to the mother's lodger, Jason Owen, 37, who could be released in 2011.
An NSPCC spokesman said: "While the indeterminate sentences passed on these two and the life sentence passed on the third for the rape of a two-year-old girl are welcome, the minimum sentences mean they could be free in a few short years.
"The sentences mean they would still be young enough to start new lives and new families when released."
The charity called for all three people jailed for Peter's death to be put under stringent monitoring on their release to ensure they never harm another child.
NSPCC chief executive Andrew Flanagan said: "We are disappointed that the minimum tariff was so low. It raises the question of how bad the abuse has to be before offenders get a longer minimum time in prison.
"Baby Peter suffered sustained abuse leaving him with horrendous injuries.
"Two of his abusers could walk free at a time when Peter should be a schoolboy with a new world in front of him. Despicable cruelty has denied him that opportunity."
Justice Secretary Jack Straw defended the sentencing system and said the Government would do everything in its power to protect children.
He said: "These were terrible crimes, unspeakable almost beyond words. The whole country has been deeply affected by the case of Baby Peter.
"It will now be the role of the prison service to monitor these offenders within prison. The judge has been clear about the minimum terms they must serve, and such decisions must be for the judiciary.
"Beyond that time they will only be released if the parole board decides after a rigorous safety assessment that it is safe to do so.
"If the parole board thinks they are not safe to release, they will remain in custody until such time as they are, whenever that may be."
Mr Straw said a "large proportion" of prisoners given life or indeterminate sentences, such as the defendants in the Baby P case, served more than the minimum period set down by the judge.
"We have greatly toughened up the sentencing system, which is one reason why prisoners are serving longer sentences and there has been a record increase in the prison population," he said.
Campaigners said serious questions remained about the failings of child protection authorities in the case.
Haringey Council in north London was severely criticised for not taking action to save both 17-month-old Baby P and the two-year-old rape victim.
Both children were on the local authority's at-risk register when the appalling crimes were committed.
Haringey Council's leader, Claire Kober, said: "We failed these children and I apologise unreservedly for the shortcomings and mistakes in our child protection service in 2007 when these crimes were committed.
"We are making urgent and substantial improvements to the service and we will act on recommendations set out in the two serious case reviews into both of these cases."
Lord Laming, who carried out a major review of child protection services after Baby P's death, said the tragedy reinforced his belief that reforms had to be implemented nationwide.
He said: "These are tragic cases, and I share the sense of shock and concern felt by everyone, not least because of the failure of each of the key public services to intervene and protect these young children already identified to be seriously at risk."
Lord Laming went on: "Being on the frontline of services to protect children from deliberate harm or neglect is tough and demanding work.
"And as I outlined in my progress report, now is the time for a major step change in the managerial accountability of these key services as well as a major change in the recruitment, training and supervision of frontline staff.
"I am confident that the recommendations I have made will be implemented fully and will produce the step change needed to improve children's services across the whole of the country."
Lynne Featherstone, the local Liberal Democrat MP in Haringey, added: "The guilty have at last been punished, but we still cannot rest until we fully understand how this poor little boy suffered so terribly, and for so long, under the noses of Haringey's children's services."
The Local Government Association (LGA) said there was still a long way to go if society was to reduce to a minimum the chances of children being abused by dangerous individuals.
LGA chairman Margaret Eaton said: "The appalling story of what happened to this little boy has shocked and saddened people all over the country, in every walk of life.
"The individuals responsible for this child's death are now being punished for what they did.
"Protecting vulnerable children is one of the most important jobs local councils do and they are committed to doing it as well as they possibly can."
Children's Secretary Ed Balls said that what happened in Haringey was "unforgivable" but insisted changes were being made up and down the country to try to prevent such a case occurring again.
He said: "What I can't do is make a commitment that I can stop adults harming children, because I don't have that power, and even though we don't understand why an adult could do this to a child, it does happen.
"But where it comes to the attention of the social worker, the police officer, the GP, the paediatrician, they should act to put the child first and not be deceived by parents covering things up when there's actual harm there right in front of you, staring you in the face, as happened with Baby Peter."
He added: "The judge was very clear here, that the mother in this case was very deceptive, she was very manipulative - she hid the evidence from the professionals.
"At the same time, the evidence was there to see, and when there was clear evidence of non-accidental injury, real harm for this child, the child's interests should have been put first.
"And there were opportunities to act, to take the child to a safe place, and they weren't properly taken, and that's wrong.
"And that's about the social workers and the police and the health service, in that particular case not doing what they should have done and letting down that little boy."