Prince George is said to have warned his classmates they “better watch out” as his dad will one day be King. A book on the royal family says the cheeky remark was made by the nine-year-old when he “sparred” with other pupils. Author Katie Nicholl said about the eldest son of parents-of-three the Prince and Princess of Wales in her new tome ‘The New Royals’: “George understands he will one day be king and as a little boy sparred with friends at school, outdoing his peers with the killer line. ‘My dad will be king so you better watch out’.” Katie added George was being raised alongside his siste…