"He's definitely a kid at heart and I think that really helps," she tells PEOPLE. "He can get into the kids' energy easily. He's fantastic with my daughter, and she just loves him."
Hannigan, 36, who has a 19-month-old daughter, Satyana, with her husband Alexis Denisof, says her costar and his partner David Burtka will be "fantastic fathers" to their twins, Gideon Scott and Harper Grace, and thinks Harris's creativity will go a long way.
"He has this phenomenal song that he has made up and it makes my daughter laugh every time he does it," she says, laughing. "It sticks in my head for days. It's so brilliant, and it's the perfect baby song. He's definitely going to be so creative and fun."
Like some new parents, Harris and Burtka sought out parenting advice and made a visit to Hannigan's home with a list of baby-related questions.
"I don't think they needed any of our advice but we peppered them with all the information that my husband and I have," Hannigan says. "We gave them a list of our favorite products and talked about [how] the first six weeks in is kind of the hardest. [Just] trying to figure out what each cry means or what is the best schedule for the baby is hard."
But with all the feedings and the diaper changing, Hannigan says Harris and Burtka will experience a new kind of love.
"Without a doubt they will be wonderful dads who are going to love their little babies so much," she says. "Those are going to be two lucky babies."
"I can't wait to share the feeling that your heart has exploded into something bigger than you ever thought it would be," Hannigan says. "It's so overwhelming in the very best possible way. I'm excited for their journey."