Councils, schools, the police, youth offending teams, and health professionals will be subject to the new regime from September 2019.
Ofsted, Care Quality Commission (CQC), HMI Constabulary and Fire & Rescue Services, and HMI Probation will look at how services work together to improve child mental health.
Inspectors will also undertake a "deep dive" into how agencies assess and support children aged ten to 15 who are subject to child in need or child protection plans, or are a looked-after child.
Ofsted said that child mental health must be prioritised in budgetary decisions - and cited NHS statistics from last year showing that one in nine children aged between five and 15 has a mental health disorder.
In each area, inspectors will look at how agencies identify and intervene early.
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Ofsted's national director for social care Yvette Stanley, said: "At a time when local authorities and their health partners are making difficult decisions about resources, it's important that the needs of children with mental ill health are being met.
"We are all responsible for children's mental health.
"We don't expect frontline practitioners to diagnose conditions, but we do expect them to be able to identify concerns and to know where to turn to for advice and support.
"These inspections will help us to see where children's mental health needs are being met and where things need to improve."
Ursula Gallagher, deputy chief inspector of general practice and children's health at the CQC, said the commission's review earlier this year had recommended national action to make sure local services work together on the subject.
"As regulators, we also need to demonstrate the joined-up working that we expect to see in local systems," said Gallagher.
"Coming together for these joint targeted area inspections, we will be able to provide an in-depth view of how agencies are supporting the mental health of children in some of the most vulnerable circumstances, and whether progress is being made to improve their care and support."
Chief Inspector of Probation Justin Russell, said that inspections of youth offending services had also found "variable" mental health support across England and Wales.
Russell added: "We look forward to working with our partners to identify good and poor practice in this area.
"We want to ensure troubled children and young people get the support they need to thrive, not just survive."
Newly published guidance sets out how the inspections will work.