Even for an established Hollywood actor, meeting a royal family member is something to get a little star-struck over.
The actor told The Graham Norton Show that he and his family had been keeping an eye out for the royals at all the usual tourist attractions, but was stunned when he encountered George and Charlotte by accident at a play park.
"Despite going to Windsor Castle, Tower of London and Buckingham Palace, we didn't see one royal," Affleck said.
"Then when it was raining, I took my youngest to one of those kids' indoor play parks.
"It was pretty empty and then I noticed this weird vibe from the other grown-ups.
"They were all very well dressed and they had earpieces and I thought, 'For a kids' place this is tight security'."
Despite the security tipping the 44-year-old that something was up, he said he took a while to catch on.
"I was the very last person to realise that Prince George and Princess Charlotte were in there playing with my kid."
However the encounter may not have been all positive, with the actor accusing the three-year-old future king of giving Samuel a cold.
"I can now tell him that he got a cold from the King of England," Affleck joked.
Affleck didn't mention if his and Jennifer Garner's daughters 10-year-old Violet and Seraphina, 7, were also at the play centre.