Some of the jokes ARE targeted at adults. For the weiters are perfectly aware that people like you will be watching it with little kids, at least on occasion. Throwing an occasional bone to an adult may be the difference between the child getting to watch the show and the adult ushering them off to something else.

As far as the who gets to be sentient and who doesn’t question: there’s probably not that much thought put into it. The writers sit around and spitball ideas at each other. They make a list of things that would be engaging for kids yet not too expensive to animate and not too long or complicated too make them lose interest. Someone probably said what about a trip to the zoo. When it came time to do the one for the zoo they said what animal should be in the zoo. What things interest children? Penguins, what do kids see them do with the penguins well they feed them. So now you need non-sentient penguins and non-sentient fish. I’m fairly certain they’re just trying to make it up as they go along and make sure they don’t alienate kids or parents too badly.