Timpson was among several MPs in child and youth-related government roles who failed to win re-election to parliament at the June 2017 general election, losing his Crewe and Nantwich seat by just 48 votes.
His new position is as part of a five-person advisory board that provides Longfield with "advice, challenge and scrutiny".
The part-time role, which is unpaid, will involve advising Longfield on how the powers of the children's commissioner can be used to best promote and protect children and young people's rights, as well as what the key issues and challenges facing children, families and the sector at large are.
He will also seek to strengthen relationships between the commissioner and the wider children's services network across the statutory and voluntary sectors.
Timpson sought approval for the appointment from the Advisory Committee on Business Appointments (Acoba) - the government committee tasked with rubber-stamping business appointments for former ministers within two years of leaving office.
It advised that, in accordance with the business appointment rules, he should not disclose or use any privileged information available to him as a government minister in his new position.
In addition, for a period of two years since his last day in ministerial office, the committee said he should not be involved with lobbying government, or any work that relates to securing the budget for the Office of the Children's Commissioner for England from government.
The committee's decision letter adds that Timpson "may take up other roles which involve working with the government in some capacity".
It said that while it was not for the committee to advise on such roles, as they cannot be considered "outside of government" under business appointment rules, should Timpson be employed by a government department, both it, and the Office of the Children's Commissioner should consider "whether any potential conflicts may arise" from his involvement with the other and "take any necessary, appropriate steps".
Longfield said Timpson is one of two new appointments to her advisory board, alongside Josh MacAlister, chief executive of Frontline - a training programme dubbed Teach First for social workers.
"They will help, along with the existing board members to advise on our work and priorities, and scrutiny of our progress," she said.
"They bring, as all the board do, different perspectives to our discussions. I'm delighted to have them join, as both are highly regarded for their commitment to improving children's lives and bring a wide range of expertise and experience to the board. I'm hoping to announce two further appointments shortly."
The other members of the board are chief executive of the government's Behavioural Insights Team David Halpern, government adviser on fostering, adoption and residential care Sir Martin Narey, and former president of the Association of Directors of Children's Services Alison O'Sullivan.