The children will attend a church service for Easter Sunday at St George's Chapel, Windsor Castle, on 9 April.
Prince Louis, who is now five, is expected to join his older siblings for the service.
The Duke and Duchess of Cambridge are teaching their children public service while keeping their childhood private.
Royal experts call it "smart" because it progressively introduces children to large crowds and formal obligations.
Prince George has previously been taken to official duties, such as watching the Euros in June 2021.