Kate, who has been royal patron of the charity since 2013, said: "Through Place2Be I have seen the benefits of offering children support for their mental health in the safety of the school environment."
"Both William and I sincerely believe that early action can prevent problems in childhood from turning into larger ones later in life."
Kate recorded her impassioned speech at Bethlem and Maudsley Hospital School in Beckenham, south London, to launch a week-long campaign by the charity.
Her video can be watched at www.place2be.org.uk/childrensmhw which also features tips and guidance for parents on children's mental health issues.
Looking radiant, the expectant mother - due to give birth to her second child in April - delivered the heartfelt message wearing a blue £250-Jaeger tile-print silk dress.
The shirt dress, which belts around the waist, giving a glimpse of Kate's neat bump, has a subtle collar and a pattern inspired by architecture in Florence and the Portuguese city of Porto.
The Duchess is patron of two charities that specifically help disadvantaged people, Action on Addiction and East Anglia's Children's Hospices as well as Place2Be.
Place2Be also conducted a British survey, which found that almost a third (30 pert cent) of parents of children aged between five and 18 admit they would feel embarrassed if their child wanted counselling in school, while more than one in five parents (22 per cent) would not encourage their child to take up counselling in school, even if they asked for it.
The charity aims to encourage parents to talk openly with their children about their feelings and help them to get any appropriate support.
The poll also found that fathers were more likely to say they would feel embarrassed (38 per cent compared to 23 per cent of mothers).
A Kensington Palace spokesman said: "As royal patron of Place2Be the Duchess of Cambridge is a committed champion of issues related to children's mental health and emotional wellbeing.
"Her Royal Highness has seen that issues such as addiction, poverty, abuse, neglect, loss and illness of family members, can have a long-lasting and traumatic impact if left unsupported.
"The Duchess of Cambridge is especially concerned with early intervention mental health support for young people, to tackle these issues at the earliest possible stage, so that children have the brightest possible futures, as they deserve."
Catherine Roche, chief executive of Place2Be, said: "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.
"We'd encourage all parents and carers to take a look at the ideas and tips on our website about how to talk to children about their mental health today."
- Daily Mail
