Baby sibling humor in webcomics

Adorable Sibling Rivalry: Baby Doug's Playful Mischief Annoys Puppy Sophie in the Latest Dog Eat Doug Comic Strip!

In the first panel of today's Dog eat Doug comic strip, Baby Doug messes with Puppy Sophie's ear. Sophie says, "Can't you go do baby things?"
In the second panel of today's Dog eat Doug comic strip, baby Doug says, "Bak!"
In the last panel of today's Dog eat Doug comic strip, puppy Sophie says, "Oh, I was not aware 'annoying quadruped co-habitants' was officially a baby thing."

I’m not sure where Doug learned that phrase.