The pair became parents to twins Gideon Scott and Harper Grace on 12 October (10).
And Harris' castmate Hannigan is adamant the actor will make a fantastic father - because he is already a huge hit with her own daughter, Satyana.
She tells People.com, "He's (Harris) definitely a kid at heart and I think that really helps. He can get into the kids' energy easily. He's fantastic with my daughter, and she just loves him.
"He has this phenomenal song that he has made up and it makes my daughter laugh every time he does it. 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.
"Without a doubt they will be wonderful dads who are going to love their little babies so much. Those are going to be two lucky babies."
The babies, born via a surrogate, are the first for the beloved U.S. TV and stage entertainer; Burtka has twins from a previous relationship.