The movie-star mom had to fly to New York City on Sunday to attend the premiere of her new HBO drama The Normal Heart the following evening, so she packed up the whole family and took them with her.
"We celebrated Mother's Day the whole day," Roberts, 46, told PEOPLE exclusively on Monday at the star-studded after-party at Manhattan's Four Seasons restaurant. "We started in California celebrating, then we got on a plane and we celebrated, and then we arrived here in New York and they all took me out to dinner. It was beautiful."
Roberts traveled with her husband, Danny Moder, 45, the film's cinematographer, and their three children, Hazel and Phinnaeus, 9, and Henry, 6.
In the film, Roberts – who said she enjoyed having the chance to work with Moder on the project – plays the fictional Dr. Emma Brookner, who in the early '80s was on the front lines of the battle against HIV/AIDS with activist Ned Weeks (Mark Ruffalo, as a thinly veiled Larry Kramer, who wrote the drama), as they fought to bring attention to a surging epidemic.
On Mother's Day, Roberts said the family gave her the day off from her usual mom duties. "Everybody was very well-organized for the day," she said.
No wiping up spills or other things moms usually have to do? "Well, one day out of the year, they can tighten the ship!" she said, laughing.
Her children apparently "tighten the ship" quite often, according to Roberts's costar Matt Bomer, who saw them when she brought them to the set.
"Between Mark's kids and Julia's kids, I'm going to have to go back to parenting school because their kids are so impeccably behaved," he told PEOPLE at the premiere. "I am wildly jealous. They are incredible parents."
One of the best gifts Roberts received that day was a Mother's Day card from her son, Henry, which had fill-in-the-blank sentences that he pulled together into a book.
"One of them said, 'My mom is happiest when … ' And he had written, 'When she gets home from work.' "
Indeed, coming home to her children after playing such an emotionally grueling role all day was a relief, she said. "When you get home and you see those cute little faces and those big eyes, you just go, 'Yes.' I feel like when I walk in the door, as soon as I see them, everything else just disappears."