The Duchess of Cambridge, wearing casual blue jeans, a stripy Breton top and sunglasses, held his hand as they walked around the edge of the field .
Prince Harry joined his brother the Duke of Cambridge in the competition, which was also watched by the Prince of Wales.
The Queen was also watching polo, dressed in pale pink, to present The Cartier Queen's Cup at Windsor Great Park, Berkshire.
The Royal Family attended Trooping the Colour yesterday to commemorate the Queen's 89th birthday.
Although he did not take part in the official celebrations, Prince George appeared on the Buckingham Palace balcony for the first time to watch the Red Arrows flying over.
It was the Duchess of Cambridge's first public engagement since giving birth to Princess Charlotte last month.
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