It is the first time Prince George has been officially seen in public in the UK since he left the same hospital as a newborn in 2013

Royal baby: Duchess of Cambridge gives
birth to daughter
2 May 2015 UK
The Duchess of Cambridge has given birth to a girl,
Kensington Palace has announced.
The princess - who is fourth in line to the throne -
was "safely delivered" at 08:34 BST, the palace
statement said.
The Duke of Cambridge, who was present for the
birth of the 8lbs 3oz (3.7kg) baby girl, brought his
21-month-old son Prince George to visit his sister
at St Mary's Hospital, west London.
The couple and their daughter will leave hospital
on Saturday evening.
They will travel home to Kensington Palace, where
Prince George has now returned.
In a statement, Kensington Palace said: "Their
Royal Highnesses would like to thank all staff at
the hospital for the care and treatment they have
all received.
"They would also like to thank everyone for their
warm wishes."
As he left the hospital briefly to pick up his son,
Prince William told the waiting crowds the couple
were "very happy".
Both Catherine and her daughter are "doing well",
Kensington Palace said. The name of the baby will
be announced in due course.
The Prince of Wales, who had earlier said he was
hoping for a grand-daughter, and the Duchess of
Cornwall said they were "absolutely delighted".