President Obama and the First Lady have met Prince George at Kensington Palace.
In pictures released by Kensington Palace, the Prince thanked the US president for his rocking horse, which he received as a present from the White House when he was born in 2013.
The prince was allowed to stay up past his bedtime to meet the US president.
The pictures were taken after President Obama and the First Lady had dinner at Kensington Palace with the Duke and Duchess and Prince Harry, as part of their four-day UK visit.
President Obama and the First Lady have been welcomed by the Duke of Duchess of Cambridge and Prince Harry at Kensington Palace, where they will have dinner this evening.
"Your majesty, mummy, I find it very hard to believe that you have reached your 90th year, and I suddenly realised the other day that I've known you since you were 22-years-old."
Prince Charles rounded off a long and busy day for the Queen with a touching tribute, before she lit the first of 1,000 beacons shining across the country to celebrate her 90th birthday.
Royal Editor Tim Ewart reports
The Queen has lit up the first in a network of 1,000 beacons set to shine across the country this evening to mark her 90th birthday.
Local authorities around the UK are hosting lighting events to create a network of flames in celebration.
Watch Prince Charles and crowds of well-wishers cheer on the monarch:
The Queen has celebrated her 90th birthday by greeting huge crowds of well-wishers.Read the full story ›
Great British Bake Off winner Nadiya Hussain has presented the Queen with her birthday cake on the monarch's 90th birthday.Read the full story ›
The Queen has thanked her fans for their birthday wishes on her official Twitter account.
The Queen said: "I send my best wishes to those who are celebrating their 90th birthday...on this shared occasion, I send my warm congratulations to you."
"I send my best wishes to those who are celebrating their 90th birthday...on this shared occasion, I send my warm congratulations to you"
The British Monarchy Twitter page was also decorated with balloons that float across the page.
The Queen waved to the crowds celebrating her 90th birthday from the comfort of her Range Rover.
She had just undertaken a walk-about where she greeted fans, stopping frequently to collect an abundance of bouquets.