Giving evidence at a joint health and education select committee hearing into children's mental health provision, children's commissioner for England Anne Longfield and mental health expert Dr Pooky Knightsmith said there is a lack of focus on boosting mental wellbeing support for pre-school children.
Their concerns follow the release last month of the government's children and young people mental health green paper, which details plans to improve support within schools but has no proposals around early years help.
Longfield told MPs that focusing on early years support is crucial to identify mental health issues at the earliest opportunity.
"I'd like to see a really comprehensive starting point that looks at children from birth and pre-birth onwards and recognises that problems develop along the way," said Longfield.
"The earlier and nearer to home it can be treated the better it is going to be for the child. I'd wanted to see that reflected [in the green paper]."
She added: "There are many points where you can identify where there are children with additional needs that they can be helped and issues prevented.
"Schools are a great place where children can get access, of course, but actually that doesn't mean that there can't be work beforehand."
Knightsmith, vice-chair of the Children and Young People's Mental Health Coalition, said: "One of the things that we think should be addressed in the Green Paper, and which is missing, is 0-5s and thinking about prevention."
Among proposals in the government's green paper is to incentivise schools and colleges to appoint a designated mental health lead, to co-ordinate school support and help children access NHS treatment. This will be backed by a £95m training package fund from 2019.
A further £215m will be made available to create mental health support teams to improve links between schools and the NHS.
But the panel raised concerns that further funding will be needed to meet the green paper's proposals and that available money may not be ringfenced.
"We worry about the money. We don't think there is enough money. We are concerned about how that is going to find the right places and how and if there is any manner of ringfencing," said Knightsmith.
Longfield wants the National Audit Office (NAO) to be called in to examine the likely cost of implementing the green paper as well as the current level of spending on children's mental health.
"We don't know what is being spent already. We don't know about any funding that is going to be introduced," said Longfield.
"One thing that I think would be very helpful would be to get the NAO to actually do a survey of funding."
A public consultation is currently taking place around the green paper's proposals but Rowan Munson, former member of the Youth Select Committee, who was also giving evidence, said he is concerned that not enough is being done to ensure children and young people's views will be heard.
"The real issue is that we are not talking to children and young people enough," he said.
"The green paper was not written for young people. It was very tricky for me to read and I have the experience of being on the youth select committee and such like."