"Those two know how to make kids, I'll tell you that right now," joked Grunberg, who costarred with Garner on Alias.
So what makes the couple -- who also have daughter, Violet, 3 -- such great parents?
"I think, just like anybody else, they are very attentive and wonderful people," Grunberg said. "If you think about those two, you know they are going to be good parents.
"They are involved," continued Grunberg. "They are just wonderful people."
Grunberg has never offered parenting advice because "they don't need it!" he added.
The actor -- who currently stars on Heroes -- recently enlisted Garner to record a PSA on his Web site, talkaboutit.org, which he created to help deal with his 12-year-old son, Jake's, epilepsy.