Princess Elizabeth Georgiana Adelaide was born to William and Adelaide, the Duke and Duchess of Clarence, on 21st January 1821 (rather than 10th December as IOTL) and due to her mother carrying her to term, she doesn't succumb to an intro-susception of the bowels at twelve weeks old.
She is born almost a year after her grandfathers death and becomes third in line to the throne after her uncle, Frederick the Duke of York and her father, but ahead of her cousin Alexandrina Victoria of Kent and her other uncles, of which only Ernest the Duke of Cumberland and Adolphus the Duke of Cambridge had born male issue.
On 26th June 1830, her father becomes King and on 20th June 1837 when she was only sixteen, she becomes Queen Elizabeth II of the United Kingdom, with a recency overseen (at the request of King William IV himself, who detested his brother, the Duke of Cumberland) by a council composed of the unlikable Duke of Cumberland, the liberal Duke of Sussex, the career soldier Duke of Cambridge and Queen Adelaide, the Queen Mother.
In a year and a half, she will reach the age of majority - What happens next?