Barack Obama has described Prince George as "adorable" after he met the two-year-old at Kensington Palace.
He also expressed his hope he would be as "engaging" as the Queen is as a lunch partner when he turns 90.
Prince George was allowed to stay up late on Friday when his mother and father hosted the US President and the First Lady at Kensington Palace.
Before he was taken to bed, George showed the couple the rocking horse they gave him when he was born.
The prince, who was wearing check pyjamas and a white dressing gown, also showed them the stuffed toy they sent him when his sister Princess Charlotte was born.
Mr and Mrs Obama had earlier had lunch with the Queen, who celebrated her 90th birthday on Thursday.
Mr Obama told an audience at Lindley Hall in central London: "I guess you all know why I came this week. It's no secret.
"Nothing was going to stop me from wishing happy birthday to Her Majesty, and meeting George, who was adorable.
"Michelle and I had the privilege to visit with Her Majesty and the Duke of Edinburgh yesterday. I can't tell you what we talked about. I can tell you that I hope I am such an engaging lunch partner when I am 90."
As the Obamas arrived, William, wearing a blue jacket and blue trousers, apologised to the Obamas, saying: "Sorry about the weather."
Kate wore a teal and maroon dress, while Mrs Obama was wearing a long camel coat over a lighter camel dress.