Kate was due to join husband William at the Tusk Conservation Awards event in London on Thursday evening, but pulled out just a few hours ahead of its start.
Kensington Palace said the Cambridges would be meeting the finalists during the afternoon, suggesting the children’s health was not the reason behind the cancellation.
A Kensington Palace spokeswoman said: “The duchess is no longer able to attend the Tusk awards due to the children.
“She will still however meet finalists at a tea at Kensington Palace this afternoon. The duke as patron will attend both.”
The duke and duchess have three children, eldest son Prince George, aged six, daughter Princess Charlotte, aged four, and their youngest child Prince Louis, who is almost 19-months-old.