Kate, the Duchess of Cambridge, is urging greater awareness of and care for children's mental health.
In the video that's just about two minutes long, Kate talks about the challenges to children's mental health -- including bullying, bereavement, domestic violence and family problems.
Without support, she says, those challenges can become traumatic, leading to mental health problems in children such as anxiety, depression, addiction and self-harm.
"The stigma around mental health means that many children do not get the help that they so badly need. This needs to change. That is why the charity Place2Be is asking us all to talk openly this week," Kate, 33, said. "We need to help young people and their parents understand that it's not a sign of weakness to ask for help."
She also said a child's mental health was "just as important as their physical health and deserves the same quality of support," noting that "no one would be embarrassed" to seek help for a child with a broken arm.
"We really should be equally ready to support a child coping with emotional difficulties," Kate, wearing a long-sleeved, belted, blue print dress and with her hair down, added.
Kate also said she and her husband, Prince William, sincerely believe early action can prevent problems in childhood from becoming more serious later in life.
"That is why we're both supporting Place2Be this Children's Mental Health Week, and I hope you'll join us," she said.
Kate is pregnant with the couple's second child, due in late April. They don't know the child's gender as they want it to be a surprise. Their first child, George, is 19 months.
Place2Be is a charity that provides emotional support to children more than 230 schools across the United Kingdom. Kate is the organization's royal patron.
According to Place2Be's website, depression and anxiety have increased among teenagers by 75 percent in the past 25 years, and three children in every classroom have a diagnosable mental health problem. The organization also said children were less likely to suffer from serious mental health difficulties later in life if they receive support at an early age.
Children's Mental Health Week officially launches on Monday.