In strolls the Cat in the Hat, a figure of anarchy and moral decrepitude. He promises meaningless frivolity to Sally and her brother, encouraging them to disregard their mother's rules -- if she's not here, what does it matter? Only the family's pet fish tries to remind the kids that adhering to a code of conduct is important, the freaking killjoy. This is a good point to mention that the image of a fish has traditionally been used to represent the church.
This explains why he asks the kids to drop money in the bowl after each sermon.
Of course, authority figure or not, the fish is still a goddamn fish. He can preach all he wants, but can't ultimately force the kids to do anything. So the Cat's shenanigans are allowed to continue, but then it turns out the fish had a pretty good point -- the house does get totally effed up. These dumb kids may as well have invited Led Zeppelin to crash there.
We'll skip over whatever gross conclusions one could draw from kids playing with their "Things."