St. James's Park is the oldes of the Royal Parks in London but is often overlooked by families (although over 5 million people still visit it each year).
It is a great location to watch the Changing of the Guard at nearby Buckingham Palace, hire a deskchair and relax as the children run around or watch the Pelican's being fed fish every day at 14:30.
Due to it's location in the centre of London, it is a great sopt to let the kids run off some energy.