Mr Jenkins (Bruno's dad) is only featured in two scenes, but in our brief time with him, we get a pretty clear idea of the kind of dude he is: rude, pushy, and full of himself. Sound like a guy who might spawn an oafy kid? We think so, too.
His interactions with Grandmamma make it pretty clear that he doesn't know what it means to respect your elders (she's an 86-year-old woman, for crying out loud – cut her some slack!). He says things like, "I don't want to go up to my room, madam. […] I'll talk where I dashed well want to, madam" (16.24, 26). He sounds more like a spoiled kid than an adult, right? (Well, except for the "dashed well want to" part.) Once he realizes that Grandmamma's not to blame for transforming his son into a mouse, he doesn't even apologize. He just turns his aggression toward the Grand High Witch, threatening to call his lawyers (19.35). (Yeah, like that's going to scar her.)
Keep in mind that, other than Grandmamma (who we clearly love) and The Grand High Witch (who isn't human), this is the adult that we know the most about in the book. Because this is such a pro-kid story, having the adults seem kind of like jerks makes sense. Mr Jenkins doesn't serve much of a purpose otherwise, but he sure makes us think twice about trusting grown-ups.