From Wikipedia:
"As of 2024, the first person under the age of 18 in the line of succession to the throne is William's eldest child (and elder son) Prince George of Wales, who is second in line to the throne after his father. If the prince were to succeed to the throne before his 18th birthday on 22 July 2031, his uncle, Prince Harry, Duke of Sussex (the King's younger son), would serve as regent (if domiciled in the United Kingdom, as required by the 1937 Act), as George's younger siblings Charlotte and Louis (currently third and fourth in line, respectively) would also be minors. In the event that Prince Harry would be unable to serve as regent (a possibility since his 2019 decision to take up residence in the United States), the next in line would be the King's brother (Prince George's grand-uncle) Prince Andrew, Duke of York, followed by the Duke of York's elder daughter Princess Beatrice."