The government pledged a raft of children's mental health services improvements as part of December 2017 green paper Transforming Children and Young People's Mental Health, backed with £300m in extra funding.
This included appointing a designated senior mental health lead in every school and college, creating mental health support teams linked to groups of education settings and introducing a four-week maximum waiting time for access to child and adolescent mental health services (CAMHS).
But a joint report by the health and social care and education select committees criticised the proposals for not being swift enough, being only available in some areas and for not taking into account financial pressures facing schools and health organisations.
The green paper was also criticised by MPs for failing to take into account fragmentation across education, as a result of the establishment of academies and free schools, and for a lack of focus on early intervention.
MPs were particularly critical of plans to set up "trailblazer" areas to test the proposals, which will only reach between a fifth and a quarter of areas by 2022/23.
They also said schools and colleges will struggle to appoint designated mental health leads due to pressures on the teaching workforce.
"The proposals put significant pressure on the teaching workforce without guaranteeing sufficient resources," the report states.
"There is also little or no attention to prevention or early intervention. The suggested speed of delivery will leave hundreds of thousands of children with no improvements in provision for several years and with possibly worsened provision if staff leave to join trailblazer areas elsewhere."
"Witnesses raised concerns that the government was 'tinkering' rather than using the opportunity to 'truly transform' the system."
MPs also said the green paper fails to specify how groups of vulnerable children, including those in care and in contact with the criminal justice system, will be better supported.
"We regard the green paper's indication that provision ‘might extend' to areas such as young offender institutions and secure children's homes as wholly insufficient in the face of considerable need," the report states.
Health and social care committee chair Dr Sarah Wollaston said: "We want to see more evidence that government will join up services in a way which places children and young people at their heart and that improves services to all children rather than a minority."
Robert Halfon, education select committee chair added: "The government must back up its warm words by taking urgent action to address the mental health issues which children and young people face today."
Children's commissioner for England Anne Longfield said the report represents a "stark warning about the scale of the crisis in children's mental health services".
"The committee is right to say the green paper is not ambitious enough. It is time for the government to set itself an ambitious deadline, with staging posts along the way, to deliver a fully joined-up system that closes the gap between spending on adult and children's mental health services, introduces proper monitoring of need and access, and invests more in early intervention so that problems are dealt with before they become critical," Longfield said.
Children's Society chief executive Matthew Reed said MPs are right to highlight a lack of specific action to support the mental health needs of specific vulnerable groups, such as those affected by sexual abuse and domestic violence.
"We would urge ministers to ensure that at least one of the proposed trailblazer schemes should be focused on the most vulnerable children," Reed said.
Sarah Brennan, chief executive of Young Minds added: "We share the committees' concern that the proposals are being rolled out slowly, over only a quarter of the country, leaving three quarters of children with no extra support."