Everybody except those with a heart of stone love to see children at weddings. They're part of the atmosphere and complete the picture; every age of a family is traditionally represented when a member of it weds, from the very oldest to the very youngest. It's the circle of life. Royal weddings always feature children; because they're required to be there!
It's a long-standing tradition within royal circles that whoever is getting married; be that a senior royal or a distant cousin, the current core Royal Family provide the pageboys, flower girls and bridesmaids. The Queen, Prince Charles, Prince Andrew, Prince William and Prince Harry all performed this duty as children, and Princess Charlotte was doing the honors for the first time at Pippa Middleton's wedding in 2018. Prince George is already a seasoned professional, having served in this capacity at three weddings, so perhaps he showed her the ropes.