By Joe Smith The Princess of Wales revealed her son’s favourite song while talking to a choir of children during an event at Buckingham Palace. Chatting to two school choirs who were performing at the event, held for her patronage Place2Be, she learned that two of the children singing were celebrating their birthdays. Kate then joked that she should be the one singing to them before sharing that “Happy Birthday” is “one of Louis’ favourite songs.” Kate shared her youngest child’s favourite tune with the reception in honour of youth mental health charity Place 2B’s 25th anniversary. At the time…