"We wanted to do something fun for the kids, and then the kids were just not into it at all," she said with a laugh in a clip of her chat with the host.
Meghan said that Archie had a dinosaur costume but only wore the outfit for "about five minutes."
"Not even five minutes," DeGeneres, who spent the holiday with Meghan and Prince Harry, chimed in. "Finally Harry talked him into putting the head on."
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Meanwhile, Lili was dressed up as a skunk, "like Flower from Bambi," Meghan described it.
Meghan, 40, also opened up about another special Halloween she spent with Harry in 2016 when he came to visit her in Toronto (with his cousin Princess Eugenie and her now-husband Jack Brooksbank!) before their relationship went public.
"The four of us snuck out in Halloween costumes to just have one fun night on the town before it was out in the world that we were a couple," she explained.
The Ellen Show Meghan Markle
Meghan said that the party had a post-apocalyptic theme, so they were covered up in "very bizarre" costumes that made them unrecognizable.
"We were able to just sort of have one final, fun night out," she said.
Dia Dipasupil/Getty Prince Harry and Meghan Markle
Meghan also shared a new update on her daughter in another clip of her appearance.
DeGeneres revealed that "Lili's now teething" before the clip cuts to Meghan with her hands up by her mouth and saying, "Anything to relieve that."
"Tequila, anything," the talk show host quipped.
"That's Auntie Ellen for you," Meghan said with a smile.
"That's why I don't have kids," retorted DeGeneres.
The Ellen Show Meghan Markle and Ellen DeGeneres
Meghan, who wore a cream top with voluminous sleeves and cutouts paired with black pants and matching pumps, also had her trusted glam team with her. Longtime friend and makeup artist Daniel Martin helped get her camera-ready alongside hairstylist Jason Low.
"We were backstage the entire time," Martin tells PEOPLE. "It's really fun and Ellen really pulled out Meghan's goofy, funny side, which everyone will be able to finally see."