Lady Louise Windsor and James, Viscount Severn have made their second appearance at the Platinum Jubilee celebrations (Picture: PA/ REX)

The Queen’s youngest grandchildren honoured the monarch’s Platinum Jubilee alongside senior royals at a special service of thanksgiving this morning.

Lady Louise Windsor, 18, looked elegant while her brother James, Viscount Severn, 14, wore a smart suit as they joined their parents Prince Edward and Sophie, Countess of Wessex for the family’s second engagement in two days.

The Royal Family were out in full force today – including Prince Harry and Meghan Markle, who were cheered by crowds at St Paul’s Cathedral after flying over from the US to join the celebration.

The Queen herself was unable to attend the event as she struggles with mobility problems and watched the event on TV.

It is rare to see Louise and James taking centre stage, with Sophie previously saying she believed it to be important for them to ‘grow up as normally as possible’.

But both were seen waving to crowds in a carriage for the Queen’s birthday parade yesterday.

They then joined working members of the family on the Buckingham Palace balcony.

Louise – 16th in line to the throne – only realised her gran was the Queen of England when she went to school, it was previously revealed.

The Earl and Countess of Wessex and their two children, Lady Louise Windsor and James Viscount Severn, arriving for the National Service of Thanksgiving (Picture: PA)

Louise was ‘shocked’ to find out her grandma was the queen, her mum previously revealed (Picture: PA)

The family have taken a centre stage role this weekend (Picture: PA)

Viscount Severn and Lady Louise Windsor ride in a carriage as the Royal Procession leaves Buckingham Palace along The Mall (Picture: AP)

Speaking in 2016, Sophie told the BBC: ‘It happened a little while ago. Well for Louise, actually, it was much more of a shock to the system. 

‘It was only when she was coming home from school and saying, “Mummy, people keep on telling me that grandma is the queen”.’

When questioned on how her 12-year-old daughter felt about her grandmother’s title, Sophie replied that she ‘didn’t understand’.

‘I don’t think she had grasped that perhaps there was only one queen’, she said.

Saying she wanted to ‘keep her children out if it’, Sophie added: ‘They’re going to have to go out and get a job and earn a living later on in life and if they’ve had a normal a start in life they possibly can get, then hopefully that will stand them in good stead.’

Lady Louise shares a close relationship with Her Majesty and had a special bond with her late grandfather, the Duke of Edinburgh, due to their shared interest in carriage driving.

There are more than two days of celebrations left to mark the Queen’s historic 70 years on the throne.

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Prince Harry and Meghan Markle flew over from the US to join the celebration (Picture: POOL/AFP via Getty Images)

Princess Anne, Princess Royal, Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge sing during the service (Picture: Getty Images)

Younger members of the Royal Family are expected to be seen once again at an open-air concert at Buckingham Palace on Saturday night.

Huge stars will make appearances and performances – including Diana Ross, Sir David Attenborough, David Beckham and Elton John.

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Queen Elizabeth II’s Platinum Jubilee 2022