Fridge pages are Spoilers Off. You have been warned.

Fridge Brilliance

Right at the start of the game, Helga mentions that there's a new doghouse at home, despite Andrew being allergic to dogs (among many other things) and never having built one. This gets put aside as you begin your adventure, though in a blink-and-you-miss-it moment you can see a fleshy tentacle slither into the doghouse. One chapter later, when you learn of the Pulsating Mass and the Mutt that it communicates through, it's easy to infer that the doghouse is placed there by the Mass.
The gag of Helga and Andrew's 'adult fun time' being the two innocently hula-hooping in their room makes sense since Jimmy, being as young as he is, probably legitimately thinks that's what Helga means by 'adult fun time'.
Mr. Cat kidnapping and murdering Cordelia shows him to be a predator in both definitions of the term; he's a cat targeting a mouse, and he's implied to have molested Cordelia before killing her.