The Duchess of Cambridge recorded the message for the Place2Be charity and reaffirmed her status as a "committed champion of issues related to children's mental health and emotional wellbeing".
Place2Be, of which the duchess is a patron, provides emotional help and support in schools and is organising the first ever Children's Mental Health Week.
In the message, Kate says that both she and the Duke of Cambridge have seen how many children struggle to cope with issues such as bullying, bereavement and family breakdown, which can lead to depression, anxiety, addiction and self-harm.
"Through Place2Be I have seen the benefits of offering children support for their mental health in the safety of the school environment," she said.
"Both William and I sincerely believe that early action can prevent problems in childhood from turning into larger ones later in life."
The video was recorded at Bethlem and Maudsley Hospital School in Beckenham, south London, to launch the week-long campaign by the charity.
Catherine Roche, chief executive of Place2Be, added: "We're thrilled to have the support of our royal patron, HRH The Duchess of Cambridge, for this important new campaign.
"By raising awareness of the benefits of getting support early for mental health issues, we hope to change attitudes and help reduce the risk of more complex and serious problems when those children reach adulthood."
Watch the message in full below: