July 29, 2016 01:00 PM

Who may be the youngest person ever to appear at San Diego Comic-Con? Make that Sarah Chalke‘s new baby girl Frankie.

With her daughter born just three months ago, the former Scrubs star couldn’t leave her alone last weekend during her appearance at the famous event.

But that’s not all baby Frankie — whose given first name is Frances — seems to be up to these days.

“She just laughed yesterday for the first time,” Chalke, 39, told PEOPLE Thursday at an event for Philips Avent line of baby products. “It was so sweet because she’s smiling now, and all of the little sweet sounds that they make just make me melt.”

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The actress is thrilled with her baby’s progression over the past few months, and even has a little help from her 6½-year-old son Charlie — who was overjoyed to find out he’d be a big brother through a scavenger hunt that Chalke and fiancé Jamie Afifi set up for him.

“He had to go around the house and collect these clues. At each clue was a letter, and then it spelled out ‘Big Brother’ at the end,” she says. “He put all the clues together and his face just lit up and he was like, ‘Am I going to be a big brother, Mommy?’ “

While trying to raise a newborn and a young son at the same time, the Mother’s Day star has found creative ways to balance quality time between the kids.

“While Frankie is so little and needs so much of your time, [I had to make] sure that some of [Charlie’s] routine stayed the same and [am] trying to involve him in time with her,” she explains.

At the end of the day, after all of the lightsaber fighting and Lego building concludes, Chalke praises her son over his amazing older-sibling skills after only three months on the job, noting, “He’s such a good big brother and it’s actually neat having that age gap because he’s excited to help out.”

But of course, the Roseanne alum fears for the day when the two siblings might stop playing nice all the time.

“When she knocks over the Lego Millennium Falcon, that’s not going to be a good day,” she jokes.

Rebecca Sloane

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