Reed, whose constituency is Croydon North, moves into education following nearly three years as shadow civil society minister.
The post has been empty since former social worker Lewell-Buck quit after defying the Labour party line during a series of votes on Brexit.
Reed said on Twitter that he was passionate about better support for vulnerable young people.
The politician has previously raised concerns about the unfair impact of cuts to Sure Start children's centres in his consituency, with 28 dedicated children's centres in Croydon in 2010, and eight left by 2014.
Delighted to be appointed Shadow Minister for Children today, I'm passionate about better support for vulnerable young people. Congratulations to @vickyfoxcroft who takes over as Shadow Civil Society Minister - thx to the many people in the sector who supported me for 3 yrs— Steve Reed (@SteveReedMP) June 24, 2019
However, the new shadow minister for children was not in parliament yesterday, his second day in the job, instead taking to Twitter from his sickbed, on the topic of LGBT children and the curriculum.
Sorry I'm not in the chamber today to oppose Roger Godsiff's repulsive pandering to bigotry, I can't believe he understands the severe harm homophobia causes to children who are LGBT or from LGBT families - sadly I'm stuck at home with food poisoning https://t.co/E8eRkD3xUc— Steve Reed (@SteveReedMP) June 25, 2019
Shadow education secretary Angela Raynor welcomed Reed to the education team.
I am absolutely delighted that colleague @SteveReedMP joins the @UKLabour Shadow Education Team as the new Shadow Minister for Children and Families. Steve brings a wealth of experience to the role and can get to work straight away holding the government to account, so pleased 🌹 pic.twitter.com/zzMppjZHVL— Angela Rayner (@AngelaRayner) June 24, 2019
In March, Lewell-Buck said it was with "deep regret" that she had resigned.
"It has been an honour to serve in this role and work with so many committed organisations and individuals who care so deeply, devoting their time and energy to making life better for children," she added.
"I want to thank all of them for their support, expertise and friendship over the past few years and I look forward to continuing to champion our children from the backbenches."
Yesterday, Lewell-Buck secured a Westminster debate on ending child imprisonment.