Both are currently in Cornwall, where G7 leaders are meeting for their annual summit. On Friday, the Queen, and other leading royals, joined world leaders at the Eden Project.
To mark the summit, Kate and Ms Biden wrote a joint article for CNN on the importance of education and care for children.
They wrote: “The two of us believe that early childhood care and education should be seen as among the defining, strategic issues of our time.
“What would happen if we really followed the science of early childhood and started focusing on the things that would make the biggest difference for children and those who guide them?
“We could transform the prospects of an entire generation.
“This is an agenda that should unite us all.”
Both the Duchess and Ms Biden have a longstanding interest in education.
They added: “We must have new conversations in our families and communities.
“We need CEOs to consider how they support the parents and caregivers in their workforces.
“And we need leaders across the world to understand that early childhood care and education is where they can make some of the most important, long-term impacts for their nations.”
The G7 meeting began on Friday and concludes tomorrow.
The first lady spent more than 30 years as a community college teacher in the United States.
It is focused on the fight against climate change, coronavirus and threats to global security.
Mr Johnson invited leaders from India, Australia and South Korea to join the summit.
On Saturday, G7 leaders will issue the Carbis Bay Declaration, designed to stop the chaos caused by the global coronavirus pandemic being repeated.
This will aim to cut the time taken to test and develop future vaccines to less than 100 days.
Kate has focused heavily on the importance of early childcare over the past decade.