The trend involved putting a giant cake in front of a 1-year-old, often while letting a professional photographer capture the hilarious mess that ensues - needless to say, the trend has now moved on to include a bath afterwards.
It's believed that Harry and Meghan enjoyed the relaxed day with their son and two dogs, who Archie is "best friends with", according to the source.
"They make him laugh and are gentle with him."
Royal expert Kate Nicholl previously revealed that Harry and Meghan were planning an "organic, sugar-free cake" as well as a splash in the pool and a call with his Cambridge cousins to celebrate his birthday.
She told OK! magazine that Meghan "loves being creative in the kitchen", but rather than baking a sugary cake, it would likely be "organic" and "suitably healthy".
It was also unlikely that little Archie would be spoiled with gifts, she said, as he "has everything he needs".
She claimed the couple would appreciate "meaningful" presents such as donations to children's charities.
