The All-Party Parliamentary Group for Children (APPGC) said the inquiry, which will officially launch in mid-September, will be informed by the findings of a survey of the children's social care workforce in England currently being conducted.
Former children's minister Tim Loughton, who chairs the APPGC, said the group is keen to hear directly from social workers about how decisions around threshold levels were made in practice.
He said children's services are under increasing pressure.
"As purse strings tighten, and children's services have to respond to an ever-widening range of challenges, is there a risk we often only help those in most dire need?" he said.
"[We want to] understand these pressures, and establish how high the bar is before services step in to help a child in need."
The thresholds inquiry follows on from the APPGC's No Good Options report into children's social care that was published earlier this year. It found evidence of later intervention and significant variation in the way children's needs were being assessed and met in the UK.
Loughton said crucially the report showed that local authorities were interpreting their statutory duties in "fundamentally different ways".
"Our inquiry will explore the causes of variation in threshold levels across children's social care services," he said.
"It will look beyond local demographics to explore the factors affecting unexpected variation in the proportion of children in need, subject to a child protection plan, or taken into care in different local areas.
"Specifically, the new inquiry will analyse the thresholds used to determine when a child qualifies for support," he said.
The survey, which is being carried out on behalf of the APPGC by the National Children's Bureau and in conjunction with the British Association of Social Workers, is specifically aimed at those who are, have been, or are training to be qualified social workers.
It will ask them to consider their experiences when making decisions about whether a child should receive various levels of support or intervention.