Dressed in a turquoise floral outfit, the Queen looked relaxed as she left Kensington Palace after spending just over half an hour with the Duke and Duchess of Cambridge's first child.
She had travelled to see William and Kate and their baby without the Duke of Edinburgh who is still convalescing at Windsor Castle following exploratory abdominal surgery last month.
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The town was where the US military and Kurdish fighters first united to defeat the Islamic State group four years ago.
MPs are expected to debate the deal on Saturday.
The duchess is expected to speak at the event, which will involve games, music and traditional Pakistani cakes.