By Sandra Westbrooke
BBC News, London
The gardens of Buckingham Palace have never seen anything like it.
Storybook characters mingled with children at the party
A black and white Jolly Roger flag has been hoisted above a pirate ship on the royal lake and a skeleton hangs from a tree.
Thomas the Tank Engine is getting up steam and Ratty and his friends from The Wind in the Willows are having a picnic.
The historic Waterloo Vase - 18 feet high and weighing 20 tons - is surrounded by red and white fairy toadstools.
On the lawn where the Queen normally greets guests during her summer garden parties, a Mad Hatter's Tea Party is in full swing.
Meet the Queen
In the midst of this fantasyland, Her Majesty is doing a walkabout, greeting some of the excited children who are helping to celebrate her 80th birthday.
The youngsters, from all over Britain, were invited through a ballot, and the fun began well before they made their way past the guards at the Palace gates.
A procession up The Mall featured Chitty Chitty Bang Bang, Noddy, Postman Pat, and Fireman Sam. In the Marble Hall, an enormous 18 ft Big Friendly Giant (BFG) waited to say hello.
On arrival, the young guests were given purple Jamie Oliver food hampers with finger sandwiches, a salad, fruit from around the Commonwealth and British strawberries.
But the centrepiece of the afternoon is a show which follows the hunt to find who stole the Queen's bag containing her speech.
Among the many stars who've worked on the production are Sophie Dahl, Ronnie Corbett, Jonathan Ross, Kelly Osbourne, Martin Clunes, Jerry Hall, Anthony Head and Patsy Kensit.
The cast of Harry Potter attended the Queen's party
Some corgis - not the Queen's - are also involved, and as stars, have their own trailers, right behind the stage.
In the end, good prevails and the Queen gets her handbag back, and is able to appear on stage to make that vital speech.
"I am delighted to have my handbag back, I do like happy endings," she says.
For the lucky guests, the afternoon is pure magic. "It's so amazing","I can't believe it!", "It's something I'm going to remember for ever and ever and ever", were some of the comments.
But many who didn't get tickets will also benefit - the party highlights a campaign to bring literature into the lives of disadvantaged and hospitalised children.
Twelve thousand books have been donated by publishers to a book pool established through the National Literacy Trust, and some of the writers who read to youngsters in the authors' tent during the party will give readings in hospitals and schools.
Harry Potter author JK Rowling and Jacqueline Wilson were amongst a group of writers at the party.