Much as they did at last year's event, both youngsters looked to be having a ball as they toured the Windsor Great Park venue with their dad.
The pair munched contentedly on ice-creams, while they also relished the chance to sit up in the cab of an RAF transporter lorry.
It was a rare public glimpse of The Queen's two youngest grandchildren, whom the 46-year-old Prince and his wife Sophie, Countess of Wessex, have been keen to keep out of the spotlight.
Since birth Lady Louise has suffered from extropia – a rare condition that causes a squint – and her parents have reportedly wanted to protect her from media attention.
On Sunday their grandfather Prince Philip, a keen participant in the annual equestrian event, looked to be having almost as much fun indulging in his favourite hobby of carriage driving.
Although the Queen's husband, who turns 90 next year, no longer competes at the five-day festival, he was clearly delighted to take a group of guests for an informal spin on its final day.