Oliver and Daniel Horne from Cirencester will present a home-made card to Prince Charlies congratulating him and the Duchess of Cornwall on becoming grandparents to the new Prince George.
The brothers will also hand the Prince of Wales a cuddly toy for the little Prince.
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Hundreds are expected to attend the service wearing pink, Maisie Duncan's favourite colour.
A bright chilly start but a fine afternoon with sunny spells for most. Mild winds will help temperatures approaching double figures.