Garner took to Instagram on Wednesday to re-post a nearly 3-minute clip from the Sussexes of Markle, 38, reading “Duck! Rabbit!” to Archie as Harry, 35, filmed the video.
In the caption, the 48-year-old actress addressed the couple, writing: "Dear Meghan and Harry, Thank you for sharing your gorgeous son with the world and allowing us to join you in celebrating his first birthday."
"We are humbled and grateful that you chose @savewithstories to mark this special day and in so doing - have raised the visibility of @savethechildren’s work in the US and UK and have helped feed and educate children in desperate need of both," Garner continued.
She added: "More than anything - watching you lovingly talk through the pictures while your clever Archie turns the pages - lifts all of us up and reminds us that what may seem like a duck just might turn out to be a rabbit. Thank you for this joyful and meaningful shift in perspective. Best, Jen."
Markle and Harry are supporting the Save With Stories campaign in the U.S., as well as Save The Children U.K., and wanted to use Archie’s birthday on Wednesday to raise awareness of having books and other learning tools for children during unprecedented times.
In addition to the reading session, a source previously told People magazine that the couple would be celebrating their son's special day -- amid California's stay-at-home orders due to the coronavirus pandemic -- with "a smash cake and Zooms with family and friends."
"It’s been beautiful weather, so perhaps a family stroll with the dogs," the insider added at the time.
Fox News' Stephanie Nolasco contributed to this report