Analysis by the Office of the Children's Commissioner into the scale of vulnerability among children in England estimates that millions of children are living "vulnerable or high-risk lives".
This includes 670,000 children in England growing up in "high-risk" family situations, 580,000 children in the care of the state, 27,000 children living with adults in treatment for drink or drugs, and 800,000 children suffering from mental health difficulties.
There are a further 46,000 children involved with gangs, 170,000 young unpaid carers, and more than 1,000 new child victims of slavery each year.
Longfield said there are many more children under the radar, because many children are not reported to services, or because of "worrying gaps" in available data.
"It is shocking that half a million children - a number equivalent to the entire population of Manchester - need direct intervention or care from the state because they are living vulnerable lives," Longfield said.
"On top of that there are many hundreds of thousands of other children growing up in potentially high-risk situations.
"Yet even more shocking is that this is only the tip of the iceberg. The actual numbers are likely to be much higher.
"The truth is nobody knows the exact number of vulnerable children. We can trace in minute detail the academic progress of a child from four to 18 and beyond, but when it comes to describing and assessing the scale of negative factors in a child's life which will hamper their progress, we are floundering.
"What we do know is that even these numbers are unacceptably high. Our ambition as a nation should be for all our children to live happy and healthy lives. This report shows that millions are not doing so - and that has to change."
Giles Meyer, chief executive of the Carers Trust said there are hundreds of thousands of young carers who are caring for a family member "while missing out on their own childhoods".
"Many are struggling to complete their homework on time, being bullied in school or have mental health problems because of the pressures of caring," he said.
"Unless they are identified, they can't be supported and this can be done with the help of local authorities, schools, health professionals and others coming into contact with young people.
"Otherwise they will continue to be failed by society along with so many other children."
Children's minister Robert Goodwill said every single child should have their voice heard and receive the care and support that they need to realize their potential.
"We recognise the scale of this challenge - and, while the number of children in need has remained relatively static since 2010, there is always more to do.
"We will look carefully at these exploratory statistics and I am looking forward to working with the children's commissioner as this important work continues."
